Selecting target in mission planner video hub to geo-locate its position

Hi everyone,
I have been trying to build a uav with geo-locating capibality of user select target in vedio hub. According to pinhole camera model, only position and attitude parameters of uav are required to calculate target location from image pixal coordinates.
The entire process will be -
(position, attitude data and video stream) are sent from uav to ground (mission planner) ----
user click the target from video hub and pixal coordinate of video frame is extracted and process internally to get target global position and sent them back to uav as way point coordinates.
How can i implement user click feature in mission planner video hub or image process in general and sent back to uav via mavlink command. Thanks.
Ps. external and internal calibration are already done for camera.