YouTube videos will embed automatically, assuming you use the right type of URL. Using the URL from the address will not always work.
Here's how to embed a YouTube video into your post:
On YouTube, right click on the "Link" or "Embed" selection and choose "Copy video URL":
Back in the forum editor, paste the URL into your message:
(Note: I modified the code to accept the shortened "Share" link, as noted in the post below).