What Squarespace variable represents the Discount Code used?

I am attempting to set up an affiliate program for my site. I am using the program Post Affiliate Pro, and in setting up the tracking of purchases, Post Affiliate Pro has given me a code to inject into the order confirmation page which will send them info if a customer used a coupon code given by a certain affiliate.

However, in the instructions they gave me, they said I would need to edit the code and replace ('Coupon_Code') with whatever squarespace uses as a variable for its coupons

Here is the code they said to modify and inject: sale.setCoupon('Coupon_Code');