If you look at the bar above where you type a reply you will see a little film icon on the right hand side of the bar. (Right next to the little quote balloon) Copy the hyper link to the video,highlight the hyper-link (then control,c keys),click the film icon,then paste the link (control,v keys) click OK and then click post reply and your done.
Gives me goose bumps to think about it. . .