Go to Youtube, pick your video then copy the address, come back here then paste the address into the 'Media' icon drop down box - click enter, you're done!
Just done a search and this guy explains it better than I did - https://not606.com/threads/posting-photos-and-videos.237215/#post-5622009
Try this, assuming you're on a pc. If you're on a phone I don't know! get your video playing in youtube click in the address bar so that it's highlighted, then right-click and pick copy. in the not606 thread go to the reply box and right click and pick paste. The address should appear, then click post reply.
Thanks.....but I'm on phone rather than PC!!!!,just as well really as i don't think many would like my choice!!!