Originally Posted by CurrentlySober

https://www.youtube.com/watch?v=X1Alc4SUqLU

See the end of your link? The /watch?v= part? The part after the = sign is the code - in this case its X1Alc4SUqLU

So just paste the code only X1Alc4SUqLU highlight it, and click the 'Wrap [youtube]' button from the top of the text entry field in advanced mode

[YOUTUBE ]X1Alc4SUqLU[ /YOUTUBE]
It looks like that WITH spaces - Without it looks like this: