No, you cannot use a microwave oven before it is fully heated up. Microwave ovens work by emitting electromagnetic waves that excite the water molecules within the food, causing them to vibrate and generate heat. The oven needs time to generate these electromagnetic waves and reach the desired cooking or heating temperature.
When you first start a microwave oven, it goes through a startup process where it generates these waves and reaches the required power level. Depending on the microwave's wattage, this process usually takes a few seconds to a minute. Some microwave ovens have a digital display that indicates when the oven is fully heated up and ready for use.
If you attempt to use a microwave oven before it is heated up, the food will not cook or heat properly, and you might end up consuming partially raw or cold food. Always wait until the microwave oven is fully heated up and ready before placing your food inside and starting the cooking or heating process.