The forum is smart enough to recognize a YouTube link. So, don’t add it as a “link”, just paste the URL in your message and the forum will show it as a video inline.
If you post it as a “link” it will look like this:
https://www.youtube.com/watch?v=ZCHo9M8bkR8
Or, if you just paste the URL inline without doing anything special at all, it looks like this: