Remove your access panel, and take out the old cord. You can use a computer power cord. Just cut off the end and strip back the outer casing. You will find a Black, White and Green wire. Just connect the wires to the same terminals in the Microwave, Replace your covers and you are set to go. (N.B. Some makers solder the ends of the wires or use crimp connectors )
Just make sure that the gauge of the new cord is the same as the older one one the microwave.