Copy the URL and paste it in the message thing. Delete the letter "s" from "https." Hit submit. Profit.
Some videos might have embed disabled -- but generally, that's what you gotta do. Make sure to remove the s! What's the link of this video in particular?
OK, I think you posted it with https at first, edited it to make it http, but by then you were editing something that was in URL tags. The best way is to start from the beginning, paste the URL, and remove the 's'.