just click that little movie screen icon in the middle right above where you type in your post.
paste in the field where the utube video posts the share button.
So, go to the utube channel and the actual video.
click share
copy that highlighted blue link that appears after clicking share. ex "https://youtu.be/9XhcewGTdMo"
click the movie icon above and where it says "source" paste in that utube link for share
the video is now embedded into your reply.