You can microwave Styrofoam.
Styrofoam is microwave safe as long as you microwave the Styrofoam for only a few minutes.
If you microwave Styrofoam longer than a few minutes then it may begin to melt.
Mostly Styrofoam is good and safe for reheating foods and drinks in the microwave for a few minutes but Styrofoam is not good for cooking food in the microwave longer than a few minutes.
I microwave food on Styrofoam plates and in take away Styrofoam containers all the time for a few minutes and it's never been an issue.
I even heat water up in Styrofoam cups for hot chocolate or even reheat coffee in the microwave in Styrofoam cups.
Styrofoam containers are not meant to withstand too hot of temperatures so microwaving the Styrofoam containers longer than a minute or so can cause the Styrofoam to melt.
Just don't cook anything in the Styrofoam longer than a few minutes and watch it and if it's starting to melt then it's been in the microwave too long.
Microwave ovens vary in power so you have to watch how quickly your Styrofoam heats up.