[CST] How to simulate an ISS ( impedance standard substrate )

Dear all,
I'm tryng to simulate a CST structure like the one you can see in the picture. The structure represent the Open standard realized with alumina on a silicon substrate.
The three pads represents the landing pads of a typical ground-signal-ground probe.
The problem is how to simulate them. Do you suggest to use a waveguide port ( as shown in the second picture ) or three lumped port, one on each pad?
Thank you in advance,
V.M.

Vsl,
Per the CST manual, if there are 3 or more conductors in proximity it is recommended that you use a multipin waveguide port. I would recommend reading this section of the manual, it isn't that dense. It is easy to find by clicking on "help" while you are in the waveguide port creation menu.