How to add z component displacement in 2D x,y plane for Solid Mechanics Module

Hi,
I want to build a simple 2D (in x, y plane) model for SH waves in Solid Mechanics frequency domain. But I am having trouble in adding my prescribed Displacement in z direction (can be added only in x and y). Though the source is in z direction, the displacement is invariant in z direction which makes the model 2D-valid.

Basically, I just want to solve the wave equation with only one dependent variables: w(x,y). But we only have options to solve it for u(x,y) and v(x,y). How can I solve w(x,y) in 2D Solid Mechanics.

Thanks.