Thanks for sharing the post and queries over ‘Ansys Learning Forum’.
I would suggest you use lattice boundary assignment to the box and Floquet port assignment for the excitation. This setup and settings can be found from below mentioned examples.
There is an open example available under HFSS example folder (File -> Open Examples -> HFSS -> Metamaterial -> Split_Ring_Resonator.aedt)
There is a step-by-step document to analyse FSS structure (Help > HFSS PDFs > HFSS Getting Started guides > HFSS Floquet ports).
This setup is used very often to study the reflection and transmission characteristic of the metamaterial, frequency selective surfaces (FSS), dielectric slabs etc. The setup models a plane wave excitation with 377 Ohm of wave impedance. The another more simplified setup is to use PEC and PMC boundary instead of lattice and apply wave port excitation. This setup would model like a parallel plane waveguide structure.
The floquete port and wave ports has de-embedding feature, which allows you to shift the measurement plane and measure the S-parameters close to the surface of dielectric slab. The s-parameters can be used further to derive material properties such as permittivity and permeability for parameter extraction.
Hope this help.