Arduino for STM32

You only need to define hardware serial in platformio.ini file:
[env:blackpill_f401cc]
platform = ststm32
board = blackpill_f401cc
framework = arduino
upload_protocol = dfu
build_flags =
-D PIO_FRAMEWORK_ARDUINO_ENABLE_CDC
-D USBCON
-D ENABLE_HWSERIAL2 ;Enable hardware serial2 <=====
-D PIN ...