Simulation Setup of Rectangular Dielectric Waveguide in HFSS

I am trying to simulate a simple rectangular dielectric waveguide in HFSS (200 - 300 GHz) and am not sure how to setup my simulation such that
the proper waveguide modes are excited.

How do I setup the ports, dimensions, etc?

Thank you,
Manish

I would draw the structure in multi-sections:
Section 1: perfect rectangular metallic waveguide (RMWG)
Section 2: RMWG to dielectric waveguide transition
Section 3: dielectric waveguide inside a very large metal box. You could alter the size of the box until effect of the metal is small enough, e.g you can check the electrical length of the whole structure doesn't vary much when you change the box size.
Section 4: dielectric waveguide to RMWG
Section 5: RMWG