Click this little icon which you will find at the top of the 'post reply' page. (It doesn't appear in the 'quick reply')
and paste the 'EMBED' youtube code (- not the 'link' code) into the pop up window.
Click 'preview' to see the youtube screen before you post
It all sounds more complicated than it is
Dedicated to MOS and kinda for global shuffle day too ;D
Reign Roknit With/Without Mask For Global Shuffle Day