the owner of a car wash gave me a free coupon wash. How do i use it?

[maserati_718]

here is the picture of the machine

http://expocad.web.smithbucklin.com/ica2/Data/EC/Event/Exhibitors/76/2d09ccd9-3129-4159-810d-5794f33ceb2a.jpg

do i put the coupon into where the money goes?
thx