When you are posting your reply, you'll see a red square button that says "Tube" just above the text box. If you select it, you will get the YOUTUBE tags added to your reply. Put the ID of your video between the tags and it will show up like Treeroot's embedded videos above.
Like this (misspelled on purpose).
where video_id is the value youtube assigns to the video, like Pb-K2tXWK4w
1976 FJ40 - green and mostly stock
1996 FZJ80 - not so stock