Site Feedback has a long thread: Have links to YouTube been disabled? - #192 by Llama_Llogophile
That should put you at a place where if you scroll up a bit (I’m too lazy to find the exact spot), @DrStrangelove has done yeoman’s work in figuring out how to post video’s and explaining how. TLDR - easiest is to do the link as the very last thing and then immediately post.