Arduino for STM32

I use the Arduino IDE for The STM32F3DISCOVERY board.
The Arduino_Core_STM32 does not support The STM32F3DISCOVERY board.
I copied from https://github.com/stm32duino/Arduino_Core_STM32.
Could you suggest to solve this issue for me?

Thank you very much.
I tried to add a new variant (board).
It is difficult for me.
Do you have another method?