Click on the "Media" tab at the top. Scroll to the bottom where it says "Media URL" Then copy->Paste the URL of the Youtube video you want to share here. If it begins with "https://" just delete the "s," and otherwise leave the URL unchanged. From there, it's pretty straightforward.
If you're posting a YT video in a regular post, just copy the URl into the text message. It will auto-format an embed.
If you're posting a YT video in a regular post, just copy the URl into the text message. It will auto-format an embed.