Oh come on, why not give it a try? Find a video on youtube that you like and see what happens when you try to post it here. The worst that can happen is that it won't work. We promise not to make fun of you lol.

Jan, it's easy.
You go to a youtube video page.
Underneath the video and its title there are a few tabs. The first is called About. The second is Share.
You click on that and it will show you a tiny link that's already selected, usually, in blue.
Copy that.
Make a normal post here, click on the Image button above, then ctrl+v will paste the link you copied just between the two image bracketed words.
You're done!