Melamine dishes can be microwaved but not safely due to the chemicals that the melamine plastic contains.
When heated the melamine plastic can leach chemicals into the food that you're microwaving.
So the melamine would likely be okay in the microwave but when you microwave food in the melamine dish it causes health issues because of the chemicals.
As long as the plastic dish is not melamine you can microwave it safely without risk of leeching chemicals into the food.
Don't microwave melamine to avoid the risk of getting sick or even deathly ill as a result of the chemicals that can get into the food from the melamine that is released when it's heated.