Sending mavlink messages from arduino uno to pixhawk (CubeOrange))

Hello,
I defined a custom mavlink message in ardupilot side. Handle and decode functions are ready. (no problem about it.)
What I want to do is very simple: How can I send the same custom mavlink message to pixhawk over the serial port.

especially I am stuck at the defining my custom mavlink message in arduino side.

summary: How can I define my own custom mavlink message header file (or generate) in arduino.