Reference taken from :
How it works
a. Tap the "Add the code to your account" button above or enter the promotion code at checkout.
b. Add $50 or more in Amazon.com Gift Cards to your cart, and complete your purchase.
c. Your $15 promotional credit will be applied to your account within two days after your qualifying Amazon.com Gift Card purchase is completed or, in the case of physical gift cards, shipped.
Offer ends at 11:59 p.m. (PT), December 22, 2019 while supplies last.
Offer has a limit of one Amazon.com promotional credit per account.