Ports: Oblique excitation

Hello,

I am trying to excite a port which is based on top of my model at the XY plane.
It needs to be polarised in the x-direction. I want to tilt it with an angle with respect the X to Z axis.

I use the following equations:

Eo: x: exp(-i*kax*x)
y: 0
z: exp(-i*kaz*z)

with variables set as: ka=emw.k0, kax=ka*sin(theta), kaz=ka*cos(theta)

what should I put in the propagation constant β?