Booking form secondary

I have partnered with a health & fitness brand that promotes businesses like mine,

The process is:

1. A user finds our listing on their site
2. There’s a coupon code displayed on the listing page
3. The user clicks the coupon promo, which takes them to our calendly form
4. They would then need to enter the code which we then identify so to give them a discount

I know calendly doesn’t have a coupon feature in the booking form, so wondering what is the best way to handle this. Would I just have a free text field? note it doesnt need to validate the code as its just for our reference.