Control the node force on the boundary

It is a 3D
cubic cell model out of a whole material,  and it is necessary to make
sure that this cell can represent the whole material so the node forece of the
corresponding nodes on the opposite surface should have the same value and
opposite direction, but I have no idea about how to control the nodes force
(maybe *EQUATION ?), because untill now all the boundary control method is about
controlling the displacement. Does anybody know how? Thank you guys so
much!