Just pasting URL
Nope.
Hmm, it’s worked for me before, in this post … but not now. Curious.
I think there was some hypothesizing that some features only work in replies, not in OPs:
Nope.
Doesn’t work for youtube anyway…
Lightbulb moment. 
Thank you.
It seems to work (Another Onebox test) when using the embed URL for YouTube.