After you have started the checkout process, the payment details window should open (see screenshot below). In this modal, there is a form reading "Enter coupon code." Simply copy and paste or type your coupon code into this form. Then, hit enter or click the red button that appears, and the coupon code will be applied to your subscription.
If you see the error "Invalid Coupon Code," this means the code was entered incorrectly or is no longer active. Try entering the code again and double check that everything about it is correct (case, letters, numbers, etc.)
If for some reason you have a code that continues to fail, please continue with the checkout process and, once completed, contact us at firstname.lastname@example.org with your account's associated email address and the coupon code so that we can apply it for you manually!