Well I can't tell you specifically for Momentum as I have not used it in 17 years; been an AWR junkie since then. But. it's an open boundary simulator like Axiem (i.e. an infinite substrate), so you have to draw an additional ground layer as opposed to using the default infinite ground boundary at the bottom/top, then change those boundaries to opens. Then explicit define those ports to terminate on your ground, then cut holes in ground as desired.
Just keep in mind, in the real world, you may be mounting your PCB with DGS in an enclosure, so you have to take into account that enclosure ground. That never seems to be mentioned in the numerous DGS article (i.e. Chinese PhD dissertations) that were flooding Microwave Journal some time ago.