You mention ADS FEM, the finite element solver. If your coupler has a planar bridge (no bond wire), I would not use ADS FEM or HFSS for this. They are general 3D solvers and can be used, but require a skilled user to get good results for planar simulation.
ADS Momentum or Sonnet are more optimized for this planar simulation task, including port calibration features, and can provide better (more accurate) results in the hands of non-EM-experts. But if you need to model the bond wire bridge and don't want to staircase-approximate that with Sonnet/Momentum, then of course you can use FEM method.