Currently, you can only redeem promo codes in the web app.
Here's how to do it:
Log in to the web app.
Click "Upgrade" in the upper-right corner.
Select the annual version of Premium or Wealth, depending on which package your code applies to.
Proceed to checkout.
Enter your code in the "Promo Code" field to apply it.
Promo codes cannot be combined with other discount offers.