You need a Facebook ID while you use some of the facebook service.
But it is quite hard to fine if you are using a facebook page. As facebook is keep updating their backend. You can NO LONGER find your Facebook admin ID in your URL now, I mean not as easy as before.
1. go to your facebook page
2. click on the profile picture
3. copy the whole url in notepad first.
for example, the URL:
4.Actually the numbers on the last part of a.xxxx is the Admin ID.
5. you can try by using the url below from Facebook developer center:
replace the number part, and try your Facebook admin ID.