I don't know how to post links, but always like the great escape.
First find the link on youtube. I use google. Click onto the link to hi light then copy the link, then come over to the reply box then Look above you to the eleventh icon. It's just to the right of the picture icon. Click onto the icon. A box drops down. Paste the link into the box, click onto OK then submit.
Can one of you explain it better?