FreeRTOS does not need porting to STM32. There are lots of demos on the FreeRTOS website, and FreeRTOS is included in the CubeMX software.
Sorry to ask the obvious, but did you read the CubeMX getting started guide or Google "STM32CubeMX tutorial" - or something like that? Googling that brings up lots of hits, including
a link on the FreeRTOS website that has an overview of the workflow.