Erik’s advice is the way to accomplish exactly what you want, but using coupling is sometimes confusing.

An approximate method to achieve what you want is to use a Remote Displacement (Behavior = Deformable) on the face, and set Rotation about X and Y to be 0, but leave the Displacement in X, Y and Z Free. This will make the nodes stay on average in the same Z plane.  If you use a Constraint Equation, then the nodes will stay exacly in the same Z plane.

Below is a force applied to the top of the small area without a Remote Displacement.

Below is the displacement with the Remote Displacement.