Send custom data using Mission planner to Companion Computer?

HI,

I want to send data to the companion computer using Mission Planner. How can I go about achieving this? Can I add a custom MAVLINK Message in MP and then send it somehow using the python scripting in MP? Does that require me to update the MAVLINK library used in APSync? Is there any other way I can send data to the Companion Computer using Mission planner?