Have a read here: Is there anyway to send manually typing messages to arduino through serial port in platformio? - #16 by NormanDunbar
And also here, further up the thread: Is there anyway to send manually typing messages to arduino through serial port in platformio? - #2 by NormanDunbar
Those should help. As indeed the whole thread will.
Failing that, pop back again.
Cheers,
Norm.