All you need to do to Post a YouTube Video is, Copy the embed code in full, post in the Thread at Go Advanced
and just paste the code in the Body of the Post.
Just like you would do to post a story or answer a question.
*** I do know that the "Old way " was in Quick Reply...and After the Upgrade, you needed to use the "Insert Video" link.
But as of right now, just use the "Go Advanced" or when you start a New Thread...
post it in the Body of the Post.
Hope that Helps