Use the opening and closing [YT] tags around the YouTube video ID:
take the full URL from YouTube
https://www.youtube.com/watch?v=gfVlI96pkao, ID is gfVlI96pkao in this example:
or use the "Insert Video" button in the post editor with the full YouTube video URL
Attachment 68424
https://www.youtube.com/watch?v=gfVlI96pkao
