Get your clip up on youtube.
Highlight it's url and copy it.
Back here in the reply box
Click on insert video (the 'film' box 2nd from the right along from B I U Font Size A etc).
Up will pop a box with:-
Enter your video clip URL below.
Right click in that box.
Press 'paste'
Then OK.
Even simpler. Find the youtube vid, copy the url and paste it into your reply.