I thought the portal url was the base url on the portal page.
I used this url with /_services/about but I get an HTTP Error 404 - Not Found.

I thought the portal url was the base url on the portal page.
I used this url with /_services/about but I get an HTTP Error 404 - Not Found.
Go to the portal administration page, under details.
The portal url is the actual site for the portal.
So for example it would look like “test.powerappsportals.com” or a custom domain.
The guide to access portal management: Portal details - Power Apps | Microsoft Docs