do you need geometrical design in HFSS? Microwave coupler can easily be simulated in ansoft designer. Also, what kind of coupler do you need? do you need quadrature hybrid, lange coupler, coupled line coupler? What level of coupling factor do you need? 3 dB, 5 dB, how much?
As Shah mentioned, you need to give those details to design a coupler.Based on the coupling required only you can decide the distance between main line and coupled line.The type of coupler and frequency of operation all these factors are necessary to design a coupler.
It is a bad idea to start coupler design in HFSS. You should know at least initial dimensions to start analysisg/optimizing in HFSS beforehand. Any MW circut simulator will provide good starting data.
I have never simulated a directional coupler in ansoft hfss. But it can be a very good topic to work out. It can be really easy to do ansoft design. Also, do you want to see electromagnetic field distribution in ansoft hfss. One more thing here, I can't understand why do you want hfss design. The reason behind is that you can easily do eletromagnetic simulation in ansoft designer using a microstrip design or strip line design. HFSS is specifically used to simulate geometries like rectangular, circular, or whichever shape. I mean to say that you can analyze electromagnetic fields there. I need to think about your question. If you can give me more specifications or please double check the question because it is not a very good question to do such simulation.