When it comes to adding custom user attributes within Azure AD, you can do this through the Azure portal and use them in your self-service sign-up user flows, or you can also read and write these attributes by using the Microsoft Graph API. For more info - Define custom attributes for user flows.
- Sign in to the Azure portal as an Azure AD administrator.
- Under Azure services, select Azure Active Directory.
- In the left menu, select External Identities.
- Select Custom user attributes.
- To add an attribute, select Add.
- Fill in the add attribute information page and create.
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