Want to change/add on mission planner frontend

Ground Control Software Mission Planner
1

Hello,
I want to know the method of how to change /add something(XYZ) in mission planner frontend.
Kindly suggest me the correct code that I should run.

Thank you.

2

Mission planner is open source and can be found in here GitHub - ArduPilot/MissionPlanner: Mission Planner Ground Control Station (c# .net)