In this month's American Way magazine Booking.com has an ad that shows the $40 cash back. Going to the link shown did not show any reference to the $40 off however. They also had the ad playing on the tv screens. The code for that differed slightly. Booking.com/40aa (or it may have been booking.com/aa40). In either event none of these URL's show the green bar - although they may offer the $40 cb later. ymmv.
Booking.com code