To apply a coupon to your Calistree subscription and get a discount, you need to make the purchase through the web app.
Go to calistree.app/account and connect to the same Calistree account as the one you use on the mobile app.
Once you are connected, select one of the membership options.
This will open a Stripe checkout page, inside of which you will be able to enter and apply your Coupon.
One the purchase is completed, go back to the mobile app to enjoy your Pro access.