This repo demonstrates FreeRTOS on a PIC32MM0256GPM064 on the PIC32MM USB Curiosity board. The demo toggles LED1 every 500ms.
I was unable to find a working demo for FreeRTOS on the PIC32MM. The MX and MZ chips are supported but it appears the MM is not (though no reason why it can't be).
I used this repo for reference as well as the official FreeRTOS examples for the PIC32MX.