On the FINAL DETAILS page, you will find the link to enter your coupon on the lower left hand side as pictured. This link may not appear if the hotel being booked does not accept coupons.
If you are booking a flight through booking.com, be aware that booking.com does not handle this transaction. A close look at the web address in your browser will reveal you have been forwarded to either travelgenio, kayak, or some other third party airline booking agent. You will need to determine which site you have been forwarded to and research how to redeem your coupon on that site. Booking.com does not accept coupons for airline ticket bookings.