You can add a coupon on the checkout page—the same page where you submit your payment.
Only one coupon can be applied per reservation. If your coupon has expired, there is no way to reactivate it. In addition, we do not honor coupons found off-site.
If you have travel credits available to use and your reservation is over the qualifying amount, the referral coupon code will automatically be applied to the reservation for you. You can't combine your travel credits with an additional coupon. Keep in mind, you won't receive your travel credits until after your friend's reservation has taken place.
For more information on referrals and travel credits, please visit our Help Center article: How do referrals and travel credits work?