You have to use the video ID, which is the 11-character string of letters, numbers, etc. in the URL that comes after the v= (and before the next & if there is one). It's the bolded part here:

youtube.com/watch?v=0pCp8g-VjOs
youtu.be/0pCp8g-VjOs

That's the part you wanna put between the [youtube][youtube] tags. Like so: