How to incorporate different baud rate to start Pixhawk using Telemetry


What is the procedure to start the pixhawk at a different baudrate than default in px4 firmware using Pixhawk3 Pro? What is the difference in procedure, if it was ardupilot firmware?

This is what I have tried as of now without
I updated the Air and Ground Module telemetry with the rate of 38400 using RFD Modem tools, as that is the telemetry I using.

I also added an extras.txt file in the location "fs/microsd/etc/"with a line saying:
mavlink start -d /dev/ttyS1 -b 38400 -m onboard -r 50000

The extras .txt file works for mavlink stream rate though.

But this procedure did not yield any result. What could I be doing wrong?