If you haven't already, you can download the app HERE
You do need to be a Spot Club member to use a promo code in the app.
Once the app is open, Click Reserve Your Spot.
Select an airport, your check-in date and check-in times, and enter in your promo code. You can only enter the promo code on this screen.
Click continue to select your facility and parking type. You can click "My Discounts" to see which available discounts are being checked. Please note that the highest discount will apply to your reservation. Discounts cannot be combined.