YouTube play in background on safari

If you're talking about Safari on macOS (and I will assume you are, since you posted this comment in the macOS > Safari sub forums) YouTube should play in the background without you doing anything at all. If it is not, make sure you don't have any extensions enabled in safari that are mucking with YouTube (turn them all off and see if that helps). If it still doesn't resolve your issue, try resetting safari or testing to see if it works on another user account on the same machine.