Yes, we can 
Look above your message box where there is a long line of icons. See those three little dots followed by a downward pointing arrow, to the right of the smilie face for picking emoticons? If you hover your cursor over those dots/arrow, it will say Insert. Click it, and five options appear in a drop-down. Select/click on the first one: Media. Paste in the url of your youtube video where it says Enter media URL, then click Continue, and your video should post
Thanks for your kind help, Magenta. I see it. It looks like the video posted anyway from my end. I'll try it again.
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