A convection microwave oven with preset power settings like low, medium, and high usually does not require preheating in the traditional sense, as you would do with a conventional oven or an OTG. Preheating in microwaves is primarily done to achieve a specific temperature, which is not applicable to microwaves with preset power levels.
In convection microwaves, the microwave mode uses microwaves to cook food quickly, while the convection mode combines microwave and hot air circulation for baking and roasting. When you select a power level like low, medium, or high, the microwave automatically adjusts the power and cooking time accordingly.
So, for baking a cake or other items in a convection microwave with preset power settings, you generally don't need to preheat the microwave. Instead, you would set the appropriate power level and cooking time for your recipe, and the microwave will start cooking immediately at the specified power level.
However, keep in mind that some convection microwaves might have a preheat option for certain specific recipes or if you want to bring the cavity to a certain starting temperature. If that's the case, refer to the user manual of your specific microwave model to understand the recommended preheating procedure and duration. But for most regular baking recipes, preheating is not necessary with convection microwaves with preset power settings.