Mazda Miata was designed without a Fuel Tank, which you have known by now.
What you need to do is, you have to empty the water from the Radiator and pour the gas on it, it has separate outlet and special hoses going to the Combustion Chamber.
What actually others do rather, is to pour gas on the engine, when it mixed with oil, it would contribute to a faster combustion, giving more acceleration to the Car.
Hope this helps.... a serious advice to a serious question.