Step 1: Go to the youtube page of the video you want to post. At the top of your browser you will see the address of the page (what is in the red box).
Step 2: Select all of this by right clicking in the box, then select all. Right click again and click copy.
Step 3: When posting a new thread or replying to a existing thread, right click in the message box and click paste on the little menu that pops up.
Step 4: Submit your message and you should then see the youtube video