Additional meeting and call-related features and issues within Microsoft Teams for business
Just went through this in a MS LIVE event test...
we found that while many other APPS or windows are recognized for sharing, with Keynote presentations, you have to choose Share Desktop, to show Keynote in full play mode...it will still fill the screen - but you can not share the Keynote Window or App...
I figured out the same and have used it recently. The trouble is that my other screen goes in black, so I lose a visual connection with the rest of the group.