You don't preheat a micro, it's instantaneous.
Microwave ovens do not require preheating. They should not be operated without food inside them.
You do not preheat a microwave. A microwave heats the food, not the oven. With nothing in the oven it will not heat up.
Regular cake mixes and recipes for cake are not made to cook in Microwave ovens. There are special deserts in packages that be cooked in the microwave.