A reposting of the instructions for embedding a YouTube video in a forum post....
In YouTube, go to advanced settings and check the "Allow embedding" option..
select and copy the youtube link (the one in the format youtube.com/watch?v=xxxx
in TM, switch to "Full Reply Screen" for your post.
click the "Enter a media tag" button, 5th from left..
click the bottom option "YouTube Video".
in the pop-up box, paste in the URL. IMPORTANT
: remove the s
in https to make it http.
The video then appears within the post (all being well)...