Membership redemption code

I’m setting up a site where a new member fills out a form (Gravity Forms) and can either pay via PayPal, stripe etc via ( wooCommerce). I’d also like an area in the form where a person can enter a unique redemption code to become a member for free that way if they pay cash in person they can still fill out the form and become a member ( wooCommerce groups). Is there a way to setup an area in the form that has a redemption code unique to that person? Is smart coupons the way to go? I’m open to other ways of doing this as well.