Use the [html][/html] tags or link button ((https://forum.twguild.org/proxy.php?request=http%3A%2F%2Fforum.twilightonalex.com%2FThemes%2Fdefault%2Fimages%2Fbbc%2Furl.gif&hash=10fa6112e901e9a5a7c41ba37fb1ad2e6e18be82)) and put the link in there.
If it's from YouTube, just use the YouTube button ((https://forum.twguild.org/proxy.php?request=http%3A%2F%2Fforum.twilightonalex.com%2FThemes%2Fdefault%2Fimages%2Fbbc%2Fyoutube.gif&hash=1811686e058efce8f132dcded3933c2b39235b5d)). You just have to put the video code in the middle of the tags.
e.g. The following YouTube link:
http://www.youtube.com/watch?v=ZsQcyhBsSjI&feature=related
would be posted here by clicking the YouTube button and copy/pasting the red portion like so:
[youtube=425,350]ZsQcyhBsSjI[/youtube]
If you want to make the player to show in HD format, use youtube=640,390.
[youtube=640,390]ZsQcyhBsSjI[/youtube]
saaaweeeeeeet