@Rav Thanks for reaching out. When you use external auth provider such as AAD or AD B2C, the first time the user authenticates and tries to sign in to the portal, they are presented with a "sign up" page where the user has to complete their first/last name and email address. This is expected behavior no option to modify it. Once the user signs up successfully by filling the details and pressing "Sign Up", the sign in should complete and subsequent login attempts won't show this sign-up page.
I suggest you use sign-in and sign-up delegation in APIM to delegate user registration. This can be a good option if you already have an existing user database or authentication system that you want to use with APIM.
please refer: https://learn.microsoft.com/en-us/azure/api-management/api-management-howto-setup-delegation
Do let me know if you have any queries.
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