nOne way to do that is by creating a new coordinate system for that plane. If you make the Z axis of that new coordinate system normal to the plane, then you would use a Displacement boundary condition of Z = 0 and set the coordinate system in the Details window to the new coordinate system instead of Global. Leave the other two directions Free. This BC would allow the nodes on that surface to move in X and Y and rotate about Z in the new coordinate system, which is what a Planar constraint does.n