More YouTube video info...
Each YouTube video has a unique ID as part of its URL, and that is the characters after the '=' sign
http://www.youtube.com/watch?v=VPHKVWCxHtI
So whenever you want to post a video you will always need to post this part
↓↓ which is always the same...
http://www.youtube.com/watch?v=
+ the videos ID, e.g. VPHKVWCxHtI which is different for each video.
If the video you want to post is part of a playlist, then copying the URL will give you the information for the playlist too and you can't post that into the forum.
The video URL with playlist looks like this...
https://www.youtube.com/watch?v=XzlLjltJElc&list=PL-ocxRrb5QEha2Gh9Bb9a-8iNw0yUESKX
...and posting that lot, won't work. You need to cut the playlist part off the right hand end. Just look for the &list part in the middle of the URL...
https://www.youtube.com/watch?v=XzlLjltJElc&list=PL-ocxRrb5QEha2Gh9Bb9a-8iNw0yUESKX
...and remove everything to the right including the &list.
Lastly, but very important, if the http at the front of the URL is https, then remove the 's'.