Mk4: Matching keys, secret number?

I was reading the Bentley and it mentioned the login for the adaptation of the immobilizer, for recoding keys is a secret number that can be found on the key fob. I have looked all over and in the key fob. As anyone had any luck with finding this. Of course this only applies to '99 cars not '00+ cars, figures. Thanks for the help.
I would have posted this in the Mk4 forum but I was afraid it would get lost, FAST!