addforce to a region

Dear all
I want to use fix addforce to a specific region in my simulation. regarding that my atoms are flowing and they will not remain in their original region, How should i do the job?

You can use the "fix addforce command" with keyword region.
fix 1 all addforce fx fy fz region
This command say you when the atom is placed in that region, the force
will be added to the atom. If your region is the entire of system, It
is not necessary to write region, because your group-ID is all.