In order to apply your promo code, you will first need to walk through the payment screens. Once you get to the review screen as shown below there is a boxed field for you to enter your promo.
If you already have a subscription, you will need to cancel it and start again by
clicking on "Start reporting rent" on your main dashboard screen. Then follow the steps above. Be sure if you are copying and pasting your promo code there are no spaces before or after the code.