https://docs.microsoft.com/en-us/azure/active-directory-b2c/b2clogin
Set redirect URLs to b2clogin.com – Azure Active Directory B2C | Microsoft Docs
Update your application
Your Azure AD B2C application probably refers to login.microsoftonline.com in several places, such as your user flow references and token endpoints. Make sure that your authorization endpoint, token endpoint, and issuer have been updated to use your-tenant-name.b2clogin.com.