Hello @Mike ,
Thank you for reaching out Microsoft Q&A.
I understand that you want to modify the authentication method in Entra ID to allow login using the Employee ID. However, this is not possible, users cannot sign in to the Entra portal using their Employee ID but can use their email ID as an alternative. From Microsoft Entra ID side, there are no settings where we can configure Employee ID as a sign in method.
For additional information follow: https://learn.microsoft.com/en-us/entra/identity/authentication/howto-authentication-use-email-signin
Hope this helps. Do let us know if you any further queries.
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Regards,
Goutam Pratti.