Hi @AmyWoods2 ,
What you are looking for might be not exactly documented. However, if you need to find that value you may try the below:
1. Logon to your ArcGIS Online organizational account.
2. Navigate to content, if you have any existing feature layer or if not you can search in your organization you should find any feature service or map service or any other ArcGIS Server services.
3. Go to the details page and scroll to the bottom of the page there will be a URL which will have the value you are looking for.
Cheers,
Manish