If you have a referral code to use with your Táve subscription, you can add it to your account in Settings › Manage Subscription. You'll click on the link at the bottom of that page that says 'Have a coupon code?'. You'll then be able to apply the code and see the price adjustment immediately.
Coupon codes will only apply the discount to the month-to-month payment term since the annual licenses already have discounts built into them. Coupon codes can only be used on trial accounts.