Did you want to check your camera and background before a Teams meeting and ended up trying to join a meeting early to do so, or worse joining a meeting late as you needed to adjust your settings and it took too long, or worst still what about joining a meeting and bringing up a totally inappropriate camera/background left over from the last meeting?
As backgrounds are persistent between meetings, use the Meet Now button to bring up your meeting join screen.
Then select your background adjust device settings and then just close the join window. When you go to join your meeting now the camera is set to go.
The Teams Queen Blog 