Q:

How to display "Working" toast from Google Docs add-on

When starting Spreadsheet or Doc add-ons I've obtained from the add-on stores, I see an animated progress toast at the bottom of the window:

That doesn't show up for my own add-ons, so I'm guessing that it's not placed by the add-on framework, but is something under the control of the add-on itself.  The Preview Guide doesn't mention it, it's not in the Quick Start, nor does it appear in the UI Style guide.  Then again, there's no elements in the CSS that would apply, either. 
How can my add-on trigger this toast during initialization? 

A:

That progress indicator actually is placed by the add-on framework, for true add-ons. ( As opposed to contained scripts using the add-on menus.) 
It is not under control of your script.