Good conductor (surface impedance) boundary conditions in FEM

Hey all

I am trying to write a simple solver for TE modes in a parallel plated waveguide in Matlab. Following Jin's annotation for the Ritz method, I am trying to implement a surface impedance boundary condition on the y boundaries, which would apply

d/dn Hz = ±j*w*e0*Zs Hz

Which according to Jianming would mean
γ = -j*w*e0*Zs
α_x = 0
α_y = 1

I am very unclear regarding how to apply this

Any hints?