No, you cannot use a microwave oven before it is heated up. Microwave ovens work by using electromagnetic waves to generate heat and cook food. When you turn on a microwave oven, it activates the magnetron, which produces microwaves.
The microwave oven needs a short amount of time to reach its operating power level, and this is commonly referred to as "preheating." During this time, the magnetron heats up and starts emitting microwaves. Once the oven is heated up, it can then transfer this microwave energy to the food, causing the water molecules in the food to vibrate and generate heat, which cooks the food.
So, it's essential to allow the microwave oven to preheat for a few seconds (usually a few seconds) before putting the food in to ensure that it functions properly and efficiently. Always follow the manufacturer's guidelines and safety recommendations when using any kitchen appliances, including microwave ovens.