You can get the stamp at the service desk in the arrivals floor, which is located in front of the taxi station, 1st floor. Fill in all your information (passport number, name, address, credit card number) in the form and leave your tax free form with the stamp in our return box.
When the office is closed, you can get a stamp from Customs, which is located in the entrance hall in front of the taxi stand, 1st floor, and at the gate 29.