If you want to simulate a patch antenna and PIN diode simultaneously, you should use a Co-simulation of the ADS.
If you don't have this tool, you simulate a patch antenna only. and then you can export touch-stone file. Using this touch-stone file, you will have to simulate circuit-simulator such as ADS, Ansoft Designer, and so on.
To do so, you have to use co-simulation in ADS or otherwise use Ansoft designer along with the data file of patch antenna exported from HFSS. Comparatively ADS co-simulation is easy to use.