I think you use the [ youtube ] button and then put the ID and then close the youtube tag
so
https://www.youtube.com/watch?v=J3mA-aVs2U8if I use the full URL becomes:
[youtube]https://www.youtube.com/watch?v=J3mA-aVs2U8[/youtube]
or if I use just the ID bit (ie. J3mA-aVs2U8):
[youtube]J3mA-aVs2U8[/youtube]