How can I record a window using ffmpeg by id?

I want to get the id of the window by name using c++ or qml and record the specific screen using this id in ffmpeg.

ffmpeg -f x11grab -wid 0x6200012 -s 1920x1052 -r 30 -i :0.0+0,0

How can I do this?

it's not necessary to be the id, can be the offset-x and offset-y, I just want to record the window in any position.