To embed a video, you only need the YouTube video ID ... shown in the last portion of the URL that is after the "/watch?v=" part at the end. See the second pic for the highlighted letters on this video ... wKsoXHYICqU.
Copy/paste that video ID into your post, highlight it and press the "YouTube" button. The "YouTube BBC Tags" will be placed around it. (You can also press the button first, and position the cursor between the YouTube tags and paste the video ID in).
And here's how it looks in a post: