Speaking as general terms, not entirely sure if this is what is going on with your device specifically.
The problem is that when you grab links, Youtube tries to be helpful and gives you the short version (more Twitter friendly and the like), so the youtu.be like ones. To get them to display here, you have to post the full link (usually by opening that short link you have on Youtube, then grabbing the full URL).
So, in this case, rather than posting the link you did, you'd attach:
https://www.youtube.com/watch?v=-RY4R3_O7RE
Or paste it into the video attachment box: