Publishing Self-Service Sign up for External Identities with PowerBI

I am trying to provide a sign up page for External Identities to access a PowerBI Dashboard. I have setup Google and Facebook federation successfully and created an External User Flow. However when publishing PowerBI as an Application I don't get the option to create an account.

https://learn.microsoft.com/en-us/azure/active-directory/external-identities/self-service-sign-up-overview

I am using the PowerBI Tenant URL and this allows me to sign in with Google but not create an account. Is there something specific around the redirect url you need to add in the PowerBI App Registration?