How to use a coupon code
When you upgrade your paper to a Pro subscription (after your 5-day trial is over), are asked to choose your Pro plan, and enter in your billing information. Near the bottom of the billing info portion, you'll see a link for "Show Options."
When you click that link, a section will appear with a checkbox for "I have a coupon to redeem."
Click that checkbox and a field will appear where you can enter in your code (see image below).