We've added a feature to the forum that will allow you to paste the URL to a web video into your post, then display the video itself so users can view it right in your message. Over 200 websites that offer videos are supported.
Since YouTube is clearly the most popular video site here is a quick guide to posting a YouTube video. Copy the URL from YouTube, stopping at the first & character (if any) do not copy the Ampersand, just everything before it. Paste it here. Preview. You should see the video in your post.
That was easy!