Stress or strain on a boundary

Hello,

I'm trying to find out what the effect of 'holes' in a hydrogel is on the modulus of that gel. For this reason, I simulated the gel in 2D as a square block. On this block, I want to apply a shear or stress on the upper boundary in the x-direction, but I cannot find the boundary condition for this. How do I apply a shear or stress on a boundary?

Kind regards