al_dddd
I have done the same thing in Momentum and got pretty good results.
I hope that your structure has two single ports for two terminals of CPWG.
If you are defining your layer as STRIP (as it is now), then use vias to connect the side grounds to the infinite ground of Momentum (///GND///). in this case you don't need to define ground reference ports.
Although there is an example in ADS example directory, examples/KC_Examples/CPW_Diff_Lines_Finite_prj in which ground reference ports for this case are defined.
second way is to define coplanar ports for SLOT layers as you said. For this one refer to the mom.pdf which should be in the folder that ADS is installed.
Hope this can help