If you have a code, scroll to the bottom and tap the "Redeem" button. Then input your code. If you have a link, simply click the link and it'll lead you to the App Store.
Your offer will automatically be applied when you open the Goodnotes app.
Promo codes for Goodnotes 6 are redeemable to anyone without an existing subscription to Goodnotes 6, including GoodNotes 5 users, Goodnotes 6 users with an expired subscription, and new users. Existing Goodnotes 6 users aren't eligible.