Add custom javascript to registration page - by: Mark McCulligh

I am wondering what is the best way to add custom javascript to the registration page. I want to add some custom business logic that if the correct information is entered I will apply a discount to the plan. My plan is to use the coupon_code input box for this.

Example: If the state/province select(dropdown) is equal to XX add coupon code XXX to the coupon code field then call the calculateSubscriptionFee method to apply it.

Therefore, just wondering that is the best way to add custom javascript to the registration form. Don't really wan to just modify the paymentmethods.js file as I know this will break future upgrades, etc...

Thanks,
Mark.