how to extract Force on a whole moving plate

Hi everyone,

Greetings! I'm a student and a new user to Liggghts. My current work is to build a model which is a plate moving horizontally in a pool full of granular particles. Now my plate can move very well, but I don't know how to extract the total reaction force on the plate, can anyone give me a hint please?
Any help is very much appreciated!

Warm regards,
Carol