To post a youtube video, open a post (hit the reply button).

Then hit the youtube button up on the tool bar. It's circled in red.

You should see that some text has appeared in your post. These are the youtube tags. Tags are surrounded by brackets "[" (open bracket) and "]" (close bracket)
Paste the url (web address) of the video that you want to post in between the two tags.
Now, remove the s from the https. It's circled in red.

Hit post and you're done!
Last, but not least, you can't post a youtube playlist, a set of youtube videos. Each video has to be posted individually.