If you want to use the YOUTUBE tags, then you must only include the part of the YouTube URL which comes after the
"watch?v=" — i.e. the
"3aDXM5H-Fuw", in this case.
The easiest way to embed YouTube videos is to simply go to YouTube, copy the entire URL from your browser's address field to your clipboard —
not the shortened "youtu.be" link that YouTube itself presents on the page with the video — and then use the VIDEO tags —
not the YOUTUBE tags, see image below — and paste the URL into the dialog box.