Check your String Rewards credit using the form at https://string.systems/rewards (skip to step 3 if you already know it).
Check your email to see your available credit (look in the spam folder if you can't find the message).
Fill the second form on that same page to request a discount coupon, up to the full amount of your credit.
Within 1-2 days, you will receive an email with your discount link 👍.