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.