Modify MAVLINK Message

Hi, I want to send a hexadecimal message using the MAVLINK protocol in order to control a gimbal, do you know where I can specifically modify the message information to include my hexadecimal string?.

Are you a manufacturer or user of that gimbal?
If you’re a manufacturer, it is a good idea to comply with the Gimbal Protocol v2 · MAVLink Developer Guide
If you’re a user, you can check whether that gimbal is supported or not by ArduPilot Cameras and Gimbals — Copter documentation
If it is not, the best practice is to implement its driver to support that gimbal in ArduPilot firmware ardupilot/libraries/AP_Mount at master · ArduPilot/ardupilot · GitHub