Hi @Rafael Caviquioli ,
Thanks for reaching out.
As mentioned in the documentation, you need to create your Microsoft Account in Azure Active Directory.
You can switch your directory from B2C directory to default directory through settings in the portal and switch to default directory to register you application in Azure AD.
While registering the application in Azure AD, you need to make sure to enter correct URL under redirect where xxxx is your B2C tenant name.
and support account type is Accounts in any organizational directory (Any Azure AD directory - Multitenant) and personal Microsoft accounts (e.g. Skype, Xbox).
Then copy the client id and client secret of your registered application to pass in Azure AD B2C tenant.
Again, switch back to your B2C directory using settings as mentioned above and configure Micrsoft Account in Azure AD B2C.
Hope this will help.
Thanks,
Shweta
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