During sign up, you can enter your promo code two ways:
Sign up at https://app.rentredi.com/signUp
Complete Account Information
Add referral code (code can be added on the first page with contact info or on the credit card payment page)
Click "Create Account"
Add payment info
If the promo code was not entered at setup and you haven't subscribed yet, the promo code can be entered while adding a payment method:
Log in to your RentRedi account
In the setup pop up, click "Choose your Plan"
Select a plan
Enter a payment method
Enter the promo code