Including external header and CPP files

Hi, what is the option in platformio.ini I need to use to include external folder CPP and H files in the current project. The external is not inside the current platformio project. The project is NodeMCU/ESP8266 based.

I tried adding -I include path to -build_flags and it didn’t work with 3.4.1a5 PIO Core

Please let me know
Thanks
Venkatesh