# EDGAR Filing Document

**Accession Number:** 0001990354
**File Stem:** 0001140361-25-037277
**Filing Date:** 2025-10
**Character Count:** 19987
**Document Hash:** 9afa2ffd19df723b1fa84567a00f153f
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001140361-25-037277.hdr.sgml**: 20251003

**ACCESSION NUMBER**: 0001140361-25-037277

**CONFORMED SUBMISSION TYPE**: SCHEDULE 13D

**PUBLIC DOCUMENT COUNT**: 3

**FILED AS OF DATE**: 20251003

**DATE AS OF CHANGE**: 20251003

**SUBJECT COMPANY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Waystar Holding Corp.
- **CENTRAL INDEX KEY:** 0001990354
- **STANDARD INDUSTRIAL CLASSIFICATION:** SERVICES-COMPUTER INTEGRATED SYSTEMS DESIGN [7373]
- **ORGANIZATION NAME:** 06 Technology
- **EIN:** 000000000
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13D
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 005-94694
- **FILM NUMBER:** 251375454

**BUSINESS ADDRESS:**
- **STREET 1:** 1550 DIGITAL DRIVE, #300
- **CITY:** LEHI
- **STATE:** UT
- **ZIP:** 84043
- **BUSINESS PHONE:** 844-492-9782

**MAIL ADDRESS:**
- **STREET 1:** 1550 DIGITAL DRIVE, #300
- **CITY:** LEHI
- **STATE:** UT
- **ZIP:** 84043
**FILED BY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** ADVENT INTERNATIONAL, L.P.
- **CENTRAL INDEX KEY:** 0001034196

**ORGANIZATION NAME:**
- **EIN:** 042840139
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13D

**BUSINESS ADDRESS:**
- **STREET 1:** 800 BOYLSTON STREET SUITE 3300
- **CITY:** BOSTON
- **STATE:** MA
- **ZIP:** 02199
- **BUSINESS PHONE:** 6179510555

**MAIL ADDRESS:**
- **STREET 1:** 800 BOYLSTON STREET SUITE 3300
- **CITY:** BOSTON
- **STATE:** MA
- **ZIP:** 02199

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** ADVENT INTERNATIONAL CORP/MA
- **DATE OF NAME CHANGE:** 19990325

## Ex-24

------

#### EXHIBIT 24

#### JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k)(1) promulgated under the Securities Exchange Act of 1934, as amended, the undersigned hereby agree that this Statement is being jointly filed, and all amendments thereto will be jointly filed, by Advent International, L.P., as the main and designated filer, on behalf of each of the entities named below that is named as a reporting person in such filing. Each of the undersigned is responsible for the timely filing of this Statement and any amendments thereto, and for the completeness and accuracy of the information concerning such person contained therein; but none of them is responsible for the completeness or accuracy of the information concerning the other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate. This Agreement may be executed in two or more counterparts, each of which shall be deemed an original, but all of which together shall constitute one and the same instrument.

Dated: October 3, 2025

---

| |
|:---|
| **Advent International, L.P.** |
| By: Advent International GP, LLC, its General Partner |

---

---

| | |
|:---|:---|
| /s/ Neil Crawford | /s/ Neil Crawford |
| Name: | Neil Crawford |
| Title: | Vice President of Finance |
| **Advent International GP, LLC** | **Advent International GP, LLC** |
| /s/ Neil Crawford | /s/ Neil Crawford |
| Name: | Neil Crawford |
| Title: | Vice President of Finance |
| **AIO Holdings, L.P.** | **AIO Holdings, L.P.** |

---

<br> By: Advent International, L.P., its General Partner <br> By: Advent International GP, LLC, its General Partner

---

| | |
|:---|:---|
| /s/ Neil Crawford | /s/ Neil Crawford |
| Name: | Neil Crawford |
| Title: | Treasurer and Secretary |

---

------

## Exhibit 99.1

------

#### EXHIBIT 99.1

#### DIRECTORS AND EXECUTIVE OFFICERS OF THE REPORTING PERSONS

The name, business address, title, present principal occupation or employment of each of the directors and executive officers of Advent International, L.P. are set forth below. Each director's or executive officer's business address is c/o Advent International, L.P., Prudential Tower, 800 Boylston Street, Boston, MA, 02199-8069. Each occupation set forth opposite an individual's name refers to Advent International, L.P. Mr. Brocklebank is a citizen of the United Kingdom and Mr. Etlin is a citizen of Brazil. All of the other persons listed below are citizens of the United States of America

---

| | |
|:---|:---|
| **Name** | **Present Principal Occupation** |
|  *Board of Managers* | *Board of Managers* |
| James Brocklebank | Director; Senior Vice President & Managing Partner; Executive Officers' Committee Member |
| John Maldonado | Director; Senior Vice President & Managing Partner; Executive Officers' Committee Member |
| David Mussafer | Chairman; Managing Partner; Executive Officers' Committee Member |
| **Name** | **Present Principal Occupation** |
|  *Other Non-Director Executives* |  |
| Amanda McGrady Morrison | Vice President & Managing Director, Chief Legal Officer, General Counsel |
| Susan Gentile | Senior Vice President & Managing Director, Chief Financial Officer, Treasurer |
| Heather Miner | Vice President & Managing Director, Chief Operating Officer |
| Heather Zuzenak | Chief Compliance Officer |
| Patrice Etlin | Senior Vice President & Managing Partner; Executive Officers' Committee Member |

---

------

### UNITED STATES SECURITIES AND EXCHANGE COMMISSION
**Washington, D.C. 20549**

## SCHEDULE 13D

### Under the Securities Exchange Act of 1934

**Waystar Holding Corp.**

*(Name of Issuer)*

**Common Stock, par value $0.01 per share**

*(Title of Class of Securities)*

**946784105**

*(CUSIP Number)*

**Amanda McGrady Morrison**<br>Advent International, L.P.<br>Prudential Tower, 800 Boylston Street<br>Boston MA 02199-8069<br>617-951-0555

*(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)*

**10/01/2025**

*(Date of Event Which Requires Filing of this Statement)*

| **CUSIP No.** | **946784105** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**ADVENT INTERNATIONAL, L.P.** | Name of reporting person<br>**ADVENT INTERNATIONAL, L.P.** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[ ] (a)<br>[x] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[ ] (a)<br>[x] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**OO** | Source of funds (See Instructions)<br>**OO** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**DELAWARE** | Citizenship or place of organization<br>**DELAWARE** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**11059899.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**11059899.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**11059899.00** | Aggregate amount beneficially owned by each reporting person<br>**11059899.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**5.8%** | Percent of class represented by amount in Row (11)<br>**5.8%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**PN** | Type of Reporting Person (See Instructions)<br>**PN** | |

---

**Comment for Reporting Person:** The percent of class is calculated based on 174,146,070 shares of Common Stock outstanding as of September 11, 2025, as reported by the Issuer in its Prospectus 424B4 ("Prospectus") filed with the Securities and Exchange Commission ("SEC") on September 11, 2025, as such amount is increased by the 16,639,920 shares of Common Stock issued in connection with closing of the Merger (as defined in Item 3).

| **CUSIP No.** | **946784105** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**Advent International GP, LLC** | Name of reporting person<br>**Advent International GP, LLC** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[ ] (a)<br>[x] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[ ] (a)<br>[x] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**OO** | Source of funds (See Instructions)<br>**OO** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**DELAWARE** | Citizenship or place of organization<br>**DELAWARE** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**11059899.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**11059899.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**11059899.00** | Aggregate amount beneficially owned by each reporting person<br>**11059899.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**5.8%** | Percent of class represented by amount in Row (11)<br>**5.8%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**OO** | Type of Reporting Person (See Instructions)<br>**OO** | |

---

**Comment for Reporting Person:** The percent of class is calculated based on 174,146,070 shares of Common Stock outstanding as of September 11, 2025, as reported by the Issuer in its Prospectus filed with the SEC on September 11, 2025, as such amount is increased by the 16,639,920 shares of Common Stock issued in connection with closing of the Merger.

| **CUSIP No.** | **946784105** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**AIO Holdings L.P.** | Name of reporting person<br>**AIO Holdings L.P.** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[ ] (a)<br>[x] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[ ] (a)<br>[x] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**OO** | Source of funds (See Instructions)<br>**OO** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**DELAWARE** | Citizenship or place of organization<br>**DELAWARE** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**11059899.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**11059899.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**11059899.00** | Aggregate amount beneficially owned by each reporting person<br>**11059899.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**5.8%** | Percent of class represented by amount in Row (11)<br>**5.8%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**PN** | Type of Reporting Person (See Instructions)<br>**PN** | |

---

**Comment for Reporting Person:** The percent of class is calculated based on 174,146,070 shares of Common Stock outstanding as of September 11, 2025, as reported by the Issuer in its Prospectus filed with the SEC on September 11, 2025, as such amount is increased by the 16,639,920 shares of Common Stock issued in connection with closing of the Merger.

**Item 1. Security and Issuer**

**(a) Title of Class of Securities:**
Common Stock, par value $0.01 per share

**(b) Name of Issuer:**
Waystar Holding Corp.

**(c) Address of Issuer's Principal Executive Offices:**
1550 Digital Drive, #300, Lehi, UT, 84043

**Item 4. Purpose of Transaction**

The Reporting Persons hold the reported securities for investment purposes, but intend to review their investment in the Issuer on an ongoing basis and may seek to increase or decrease their position in the Issuer by purchasing additional Common Stock and/or other equity, debt, notes or other financial instruments related to the Issuer (including, for this purpose, any rights or securities exercisable or convertible into securities of the Issuer) or by selling or otherwise disposing of some or all of the reported Common Stock, other equity, debts, notes or other financial instruments (which dispositions may include distributions of some or all of such securities to such Reporting Person's respective members, stockholders, partners or beneficiaries, as applicable, transferring Issuer securities to affiliated transferees, or the entry into a total return swap, asset swap or repurchase transactions) from time to time, in each case, in open market or private transactions, block sales or otherwise. The Reporting Persons may also engage from time to time in ordinary course transactions with financial institutions with respect to the securities described herein. 

Any such transactions that any of the Reporting Persons or their affiliates may pursue may be made at any time and from time to time without prior notice and will depend on a variety of factors, including, without limitation, the price and availability of the Issuer's securities or other financial instruments; an ongoing evaluation of the Issuer's business, financial condition, operations and prospects; general industry and economic conditions; the securities markets in general; tax considerations; the Reporting Persons' or such affiliates' trading and investment strategies, other investment and business opportunities, applicable legal and/or contractual restrictions and liquidity requirements; and other factors deemed relevant by such Reporting Persons and such affiliates.

**Item 5. Interest in Securities of the Issuer**

**(a)**
The information set forth in rows (11) and (13) of the cover pages of this Schedule 13D is incorporated by reference into this Item 5(a).

The reported securities are directly held by AIO, which represents approximately 5.8% of the outstanding Common Stock based on 174,146,070 shares of Common Stock outstanding as of September 11, 2025, as reported by the Issuer in its Prospectus filed with the SEC on September 11, 2025, as such amount is increased by the 16,639,920 shares of Common Stock issued in connection with closing of the Merger.

Advent LP is the general partner of AIO and Advent GP LLC is the general partner of Advent LP. Each of Advent LP and Advent GP LLC may be deemed to have voting and dispositive power over the shares directly held by AIO. 

Pursuant to Rule 13d-4 of the Act, the Reporting Persons declare that filing this statement shall not be construed as an admission that any of the Reporting Persons are, for the purposes of Section 13(d) and/or Section 13(g) of the Act, the beneficial owner of any securities covered by this Schedule 13D, and such beneficial ownership is expressly disclaimed by the Reporting Persons.

**(b)**
The information set forth in rows (7) through (10) of the cover pages and Item 5(a) of this Schedule 13D is incorporated by reference into this Item 5(b).

**(c)**
Except as set forth in Item 3 of this Schedule 13D, the Reporting Persons have not effected any transactions in Common Stock within last 60 days.

**Item 6. Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer.**

Stockholder and Lock-Up Agreements

In connection with entry into the Merger Agreement, on July 23, 2025, the Issuer entered into a Stockholder and Lockup Agreement (the "Stockholder and Lockup Agreement"), effective upon the Closing Date, with certain affiliates of Advent LP, the chief executive officer of Iodine (the "Iodine CEO"), and certain other equityholders of Parent which includes, among other things, (i) restricting the transfer of the Common Stock to be received by them in connection with the Merger for a period of 18 months after the Closing Date for Advent LP and the Iodine CEO and (ii) that on the terms and subject to the conditions set forth in the Stockholder and Lockup Agreement, from and after the Closing Date, Advent LP will be entitled to nominate a director to be appointed to the Board until such time as Advent LP and their permitted transferees no longer beneficially own at least 5% of the then-outstanding shares of Common Stock. On the Closing Date, the Issuer amended its stockholders agreement to increase the size of the Board from twelve (12) directors to thirteen (13) directors and appointed Lauren Young pursuant to Advent LP's nomination right (such amendment, the "Existing Stockholders Agreement Amendment").

Joinder to Registration Rights Agreement

In connection with entry into the Merger Agreement, on July 23, 2025, the Issuer entered into a Joinder Agreement to the Amended and Restated Registration Rights Agreement, dated as of June 10, 2024 (the "Joinder Agreement"), with Advent LP, which provides for, among other things, that 18 months after the Closing Date, (i) the right to initiate up to two short-form demand registrations and/or shelf take-downs, (ii) certain piggyback registration rights of Advent as a holder thereunder and (iii) an obligation of the Issuer, upon the request of Advent, to amend its existing shelf registration statement to include shares of Common Stock received by Advent LP in the Merger.

The foregoing descriptions of the Stockholder and Lockup Agreement, Existing Stockholders Agreement Amendment, and Joinder Agreement are qualified by reference to such agreements, which are attached hereto as Exhibits 99.3, 99.4 and 99.5, respectively, and incorporated by reference herein.

### SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**Reporting Person:** ADVENT INTERNATIONAL, L.P.

**Signature:** /s/ Neil Crawford

**Name/Title:** Neil Crawford / Vice President of Finance

**Date:** 10/03/2025

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**Reporting Person:** Advent International GP, LLC

**Signature:** /s/ Neil Crawford

**Name/Title:** Neil Crawford / Vice President of Finance

**Date:** 10/03/2025

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**Reporting Person:** AIO Holdings L.P.

**Signature:** /s/ Neil Crawford

**Name/Title:** Neil Crawford / Treasurer and Secretary

**Date:** 10/03/2025