verifying registration key in CreateUserWizard?

I followed the tutorial in the learn section, using the asp.net database given by asp.net website admin tool and using create user wizard to create new users.
 
now I have a problem. I need to add one item in the user registration: Registration Key, this is going to be stored in the database and being used by other program. I do not know how to override the CreateUser function and have it interact with the sql database. Can you tell me how? thanks.
 
another question that follows the my problem above. say if my registration key is stored in the database before the user registers, how do I do it? I figure every table in the default sql database always have user ID as primary key so I can't add an entry with user id left blank. Basically what I want to do to manually store the registration key 1st, when user registers using that key, the function checks if the key is correct, and then allow the user to register.

 2 ta ...