To obtain and use a post processor for PowerMill 2020 , you must use the Autodesk Manufacturing Post Processor Utility (AMPPU)
You have three options:
A post processor in Powermill is a software component that translates the toolpath data generated by the CAM software into a machine-specific code, known as G-code. The post processor is responsible for converting the toolpath data into a format that can be read by the CNC machine, allowing it to execute the machining operations. Powermill 2020 Post Processor Download