Most people would just put water in the mug (ceramic/microwave-safe of course) that they’re going to make the tea in and microwave it until it boils or bubbles just short of a rolling boil, which takes 2 or 3 minutes, depending on the microwave’s power–you’d learn the time yours takes and set the timer for that. At that point I don’t see the difference between that vs. if you poured it into the cup from the kettle. Either way you now have a cup full of boiling-hot water to steep your tea in. No, it won’t spill over if you don’t fill it all the way up to the brim.
Most people would just put water in the mug (ceramic/microwave-safe of course) that they’re going to make the tea in and microwave it until it boils or bubbles just short of a rolling boil, which takes 2 or 3 minutes, depending on the microwave’s power–you’d learn the time yours takes and set the timer for that. At that point I don’t see the difference between that vs. if you poured it into the cup from the kettle. Either way you now have a cup full of boiling-hot water to steep your tea in. No, it won’t spill over if you don’t fill it all the way up to the brim.