There is no special markup for youtube videos, but just adding a link should work: try
The description part is optional but a good idea.
But the link just displays the decription with a little globe icon right. It would be nice to actually have the video imbedded in the page like images are.
But this is usually a restriction on the Wiki software that you have in place. So I understand if this is not technically possible.
The userbase.kde.org wiki should install https://github.com/Whiteknight/mediawiki-embedvideo/wiki/
Are you saying you installed the EmbdeVideo extension? Because that should mean the syntax described [here] should get me an embeded video.
But no video. How about you. Does it work for you. Needs testing/troubleshooting.