Thank you for posting this in Microsoft Q&A.I understand you want to hide the UPN of the user when signing the azure login window.
Unfortunately, there isn't a way to remove or hide the preferred username (which typically includes the "guest" keyword) from being displayed during the sign-in process in Entra ID through parameters passed during the login request. As of now only users can use Email or UPN to sign-in to Azure/Microsoft Entra ID.
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.
If this answers your query, do click Accept Answer
and Yes
for was this answer helpful. And, if you have any further query do let us know.