You can now post Youtube videos directly in forum posts!
A new Youtube button should now appear in the editor toolbar. But technically you don't need this.
To post a Youtube video, just copy and paste the Youtube video URL on a separate line in your post and it will be automatically converted into the video.
Remember do not paste the embed code (it won't work), paste the URL. The URL is provided on the Youtube video page when you click the Share button.
See Section 8 of the Forum Rules.