mission planner connect problem

hello everyone, I have deigned my own custom flight controller board using STM32F7. I have modified existing mRoX21-777 hwdef.dat file as per the ardupilot suggest. I build and flashed new bootloader and boards LED also blinking. then i used the bootloader to upload the firmware with Mission Planner.It is detecting the USB in windows device manager but the problem is connecting to Mission Planner.

Can i use any APJ_BOARD_ID for this device ? Could you please suggest any necessary changes ?

Start with the modified mRoX21-777 hwdef.dat and the APJ_BOARD_ID used for the custom STM32F7 board, then review how Mission Planner handles the uploaded firmware and connection. Reproduce the bootloader upload and connection attempt; done means the custom board is recognized and connects successfully, with the required board configuration identified.

Written by the indexing model from the issue text.