Registering an app in entra (ex azure I guess?) - issues with domain

Hello - so we have a partner account, with it's own MPN ID - and when we've originally registered it it was under a domain, let's call it a.com.
Now in entra I'm trying to register my app (it will require user & calendar data via API), here:

We are enrloed, I am signed in with an account that is authorized to make changes to the organisation's Partner Center account, & I've added domain b.com in entra id and did DNS verification to it.
When I enter our MPN ID I get this:

What else do I need to do? AI agents keep telling me to do changes in entra - but I think I need to somehow add the domain b.com in the partner portal? But I don't see a place where I can add it.
What am I doing wrong?

Thank you so much for your answers.