How can I add new flight mode and custom stil download link for simulation on mission planner

Ground Control Software Mission Planner
1

I want to modify mission planner source code and add a new flight mode, replace STIL download link to my custom STIL simulation source code. please let me know what changes, that i have to perform in the source code.

2

just build sitl, and replace the MP downloaded version, and check the “skip download” box inside MP
image
check here for files to replace
C:\Users\<username>\Documents\Mission Planner\sitl