Build a model of the waveguide, add any properties of materials CST doesn't know, use waveguide ports, and CST will predict its performance.

It may be necessary to use shorting ground links to enforce the mode you want, for instant in coplanar waveguide.

Hello,

Yes, you can model any waveguide structure in terahertz range as CST has option to simulate the structure for very very small parameter like dimension of waveguide structure.

Thanks,