Discount codes (coupons) must be used on the website - you can not use them in the checkout flow with Apple or Google.

When you get to the purchase screen for PRO on the web (make sure you are logged in first if you already have a free account with us), before you check out, there is a link under the price that says “I have a discount code”.  Click that, enter the code, and then proceed with your purchase and enjoy PRO!  

If you are entering the code for a trial, the discounted price will not show up until you enter the paid phase of the subscription. 

If you cancel your subscription before the trial is over, the discount will not apply since you never entered the paid portion of the subscription.