OptimizeMember has a way for you to use coupon codes in your Pro Forms. To setup coupons, you may navigate in your site to “OptimizeMember >> Pro Coupon Codes”
You can find all instructions there pertaining to using the coupons with pro forms.
NOTE about expiry dates on coupons:
If setting expiration date for coupon, the timezone for coupons is always going to use UTC/GMT time, so you may need to adjust the expiration to consider your local time zone versus UTC/GMT and adjust as needed (in some cases if setting date for expiry to something like 2-22-2020, you may need to make it 2-23-2020 in order to ensure that it will not end prior to when it is supposed to in the time zone where you are using the coupon.