I work for a company that has recently been acquired by another company. Both companies have O365 tenants and we are working to move some services to the new company. The problem I have is that my Teams account is in a tenant other than the one associated with my Windows AD logon. I logon to my machine in Company A - user@CompanyA.com. My Teams account is in Company B - user@CompanyB.com. We have disabled the O365 features in Company A and now use everything in the Company B tenant. There is no trust between the domains, but we can access resources in the other domain using the proper credentials. All office 365 features work perfectly in Company B, except for teams. Every time I launch Teams, it wants to logon to Company A. How can I change Teams to logon into Company B instead? I was told by a previous MVP that it is not possible. I'm hoping he is wrong 😉

DaGbyte Hi David, would it be an alternative to prevent the automatic population of the UPN? If applicable you can use this reg key. Turn off pre-population of the UPN:Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\TeamsSkipUpnPrefill (REG_DWORD)0x00000001 (1)

Change Teams default login | Microsoft Community Hub

ChristianBergstrom
Silver Contributor
Oct 09, 2020
DaGbyte Hi David, would it be an alternative to prevent the automatic population of the UPN? If applicable you can use this reg key.

Turn off pre-population of the UPN:
Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\Teams
SkipUpnPrefill (REG_DWORD)
0x00000001 (1)
- DaGbyte
  Copper Contributor
  Oct 09, 2020
  ChristianBergstrom - That seems to have done the trick! I do want to point out that the HomeUserUpn value IS set correctly to my Company B address. When I added that key you suggested, it just took me right into Teams.
  - ChristianBergstrom
    Silver Contributor
    Oct 09, 2020
    DaGbyte Glad to hear it solved the issue! It would be interesting to hear what Andy asked about though. Did you try to sign out and back in manually from the Teams client, and was it still trying with the wrong UPN then?
    
    By the way, this is good one I just want to attach while we are talking about these settings.
    https://docs.microsoft.com/en-us/microsoftteams/scripts/powershell-script-teams-reset-autostart
- Jackie Behrbom
  Copper Contributor
  Dec 09, 2020
  Thanks for this! Do you happen to know if there is a regkey to make it use the user's e-mail address by default instead of UPN? We also have non-routable UPNs so stopping it from pre-populating the UPN is definitely helpful. I'm just wishing there was a way I could pre-populate the e-mail though.
  - ChristianBergstrom
    Silver Contributor
    Dec 10, 2020
    Hi, I am afraid not. But they do recommend that upn and primary smtp match. Teams will always default to the domain-joined account if one sign out och restart.
    
    You can read a bit more about upn here https://docs.microsoft.com/en-us/azure/active-directory/hybrid/howto-troubleshoot-upn-changes
    
    And if you haven’t seen these
    
    https://docs.microsoft.com/en-us/microsoftteams/sign-in-teams
    
    https://docs.microsoft.com/en-us/microsoftteams/scripts/powershell-script-teams-reset-autostart
- Haith1895
  Copper Contributor
  Feb 04, 2021
  ChristianBergstrom i do have the same issue, how do i access this location/option
  Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\Teams
  SkipUpnPrefill (REG_DWORD)
  0x00000001 (1)
  - ChristianBergstrom
    Silver Contributor
    Feb 04, 2021
    Haith1895 Hello, How to open Registry Editor in Windows 10 (microsoft.com)
    
    If you're not familiar with the registry I recommend that someone assist with the setting.
Andrew Hodges
Bronze Contributor
Oct 09, 2020
DaGbyte

This is launching the desktop client? You have Single Sign on setup?

What happens when you sign out in the Teams client and login with the other account then restart the computer?
gericht
Copper Contributor
Nov 25, 2021
DaGbyte I do face that problem running linux teams.
every time I start up Teams it asks me if it should change to a Guest account with an external organization I had been invited to some time ago, and which I have to use occasionally.
since a few weeks (after I had to interact by this guest account more frequently) this inconvenience popped up.
and I wonder - how to get rid of it?
M___S
Copper Contributor
Feb 17, 2022
I was finally able to log on with a different (pre-existing) account than the one that somehow got defined on a new laptop by unchecking, in the settings, "Auto-start application."

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