Automatically change user role

Forum team,

First, I am grateful for such a wonderful plug in! It has really helped me create a positive user experience.

Question: How can I automatically change a WordPress user role based on a S2Member custom registration field?

Example: User creates and account and puts in a specific “code” to my S2Member custom registration field, they now will automatically get a higher role based on if the code is correct or not.

The specific example can be found here: http://www.getfitstayingfit.com/wp-login.php?action=register

I am not a coder so arrays, etc. do not make much sense to me.

Thanks for any and all guidance,
Matt