Manually, you type in "https://www.youtube.com/watch?v= the jibberish code after the equals sign in the youtube URL [/yt ]" (minus the spaces)
Example:
Youtube URL - http;//www.youtube.com/watch?v=sSVuf7VoxpQ
https://www.youtube.com/watch?v=sSVuf7VoxpQ[/yt ] ← delete that space and you get...
https://www.youtube.com/watch?v=sSVuf7VoxpQ
Automagically, in your response message you skip to below the type field, click the "Go Advanced" button, then click that black/white/red YT button and paste in the video's code.
When you click the "QUOTE" button you can see exactly how it's done.
It only works for embedding youtube videos.
For vimeo we can only post the link.