One of our customers migrated from Webapp builder apps to ExB apps. Some of their apps require authentication, and they noticed that the AGE login screen is missing the "Create new user" option when redirected to from an ExB app. ("Opprett en konto" in Norwegian in the screenshot.)
The first screen is when redirected from a Webapp builder app, the second screen is when redirected from an ExB app.

I have played with the params in the URL and it appears to be client_id that is responsible.
This is generated by Webapp Builder, the create-user option shows
../sharing/oauth2/authorize?client_id=arcgisonline
This is generated by ExB, the create-user option does not show
../sharing/oauth2/authorize?client_id=experienceBuilder
The customer would like the create-user option to be on the login screen.
The "Allow users to create new built-in accounts" in Portal - Organization - Settings is ON.