New Teams is not prefilling UPN upon first start

Paul-Christiaan Dikswe are working around this by deploying both the Classic Teams with autostart set and New Teams, and setting a Teams Update Policy of either Microsoft Controlled or New Teams Default. When the user first signs-in:

1. Teams Classic starts and autofills their UPN
2. After the user enters their password, Classic Teams starts, and immediately tells the user the next time they start Teams, New Teams will take effect.
3. When New Teams starts their UPN is autofilled as Teams Classic took care of that.

In summary, we're using the autostart functionality of Classic Teams to bootstrap authentication for New Teams.

It's a hacky workaround but it seems to do the job. Judging by the below rather confusing Teams migration graphic Microsoft have released, it seems they will support this "just install both and let Teams migrate from Classic to New" approach as there's an extensive period from mid-May to end of June there where the Classic Teams installer remains available despite New Teams Only being the only operating paradigm. Hopefully they figure out this mess by then.

source: https://learn.microsoft.com/en-us/microsoftteams/teams-classic-client-end-of-availability