The purpose of the planned 5 min long night flight was to reward one of the operator's customer's employees for their sales performance record. The flight involved a view of the city. The pilot had earlier made a similar flight, and during takeoff he had turned off the helicopter's landing light to 'amplify the visual effect' of the runway lights. The helicopter's manufacturer had designed the light switch to be located adjacent to the hydraulic test switch. Upon depression of the light switch the helicopter's light turned off. Upon depression of the hydraulic test switch the hydraulic boost system deactivated, the controls stiffened, and a warning horn sounded. During the aerial taxi for takeoff, between 25 and 50 feet above ground level, the pilot inadvertently mistook and depressed the hydraulic test switch for the light switch, and the warning horn activated. When the pilot released the collective control to silence the horn, the collective lowered and the helicopter descended until touching down hard on the taxiway.