I know how :3
Go to your video on youtube and copy the url of the page that the video is on. Then, back in the forum reply, go to the little icon right over where you write your replies. It'll say "Video" when your mouse hovers over it, and it looks like a little bit of film. Press the button, paste the url into the little box, press okay, and viola!! The movie should appear.
I hope that this works, and isn't too confusing of an explanation.
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