Copy+pasting this here from
https://microsoftteams.uservoice.com/forums/555103-public/suggestions/35405383-stop-making-teams-open-on-startup so I can find it back. Confirmed to work on Windows 10 Pro.
CTRL+Shift+ESC
Startups
Right click on Microsoft Teams
find file Location
Change .exe to other name
OK
Its hard to find good help I am regularly saying that its hard to find good help, but here is