Are you using a premixed synthetic sample? If so, it can be microwaved for up to 10 sec.
As far as human samples, I dont recommend using a mic because human urine is bio-hazardous and should not be in an area where food and utensils are involved. Instead, hot water can be used to heat the sample.

A vehicle heater is also a good alternative.

The acceptable temp range is 90-100F. Shoot for 96-98F to minimize hassles.