Some types of plastic are safe to microwave while other plastic might melt in the microwave.
Most plastic dishes I've used and still use have been safe in the microwave even without it saying it was microwave safe.
Usually on the bottom of the plastic dishes they will say whether or not they are microwave safe.
If it doesn't say it's microwave safe it doesn't always mean it's not microwave safe and can sometimes still be microwaved.
Just keep an eye on the dish in the microwave in case it starts to melt somewhat.
If it starts to melt in the microwave then you need to remove it from the microwave before it melts any further and that means it's not microwave safe.