The program part that interprets links messes up with youtube links.
You have to put them on a separate line and often separated by a blank line.
Then it will put a player in your comment.

The better way is to put it into an href
<A href="https://www.youtube.com/watch?v=28HfAI2Ka3U">https://www.youtube.com/watch?v=28HfAI2Ka3U</A>
You can replace the second url with the title, or whatever text you would like to have highlighted.