Custom subdomain is domain that is part of your domain (e.g., example.yourdomain.com).
Custom subdomain can only be created with domain purchased in or delegated to Site.pro.
Go to My Websites page;
Click next to domain you want to create subdomain for;
Select "DNS Zone";
Click "Add field".
Write subdomain name in "Host" column;
Select "A" or "CNAME" in "Record Type" column;
Write A or CNAME record in "Record" column.
Note: Subdomains, except for purchased .site.pro subdomains, cannot be used for new websites.