how to use HFSS to simulate such structure??

two 10cm*10cm parallel metal plane with a 0.4mm FR4 substrate, and one metal plate have a 4cm*4cm split with it width 1mm, the split center poit is (4.5cm,4.5cm), coax feed at (4.5,4.5) and recieve is at(1.5,7.5)

I think describing the method in writing is very difficult, why don't you send a draft draw, so that we can describe to you how each part could be drawn

ya, a rough sketch would be clear to understand

can you send your sketch ? I am also simulating a similar structure.

mentioned struacture seems simple to draw. you should explain more where is your difficulty ? do have problem to drawing your structure or excitation or boundaries , etc...

If i understand you correctly,

Looks like you can set-up airbox sorrounding the 10x10cm plate and assign waveport at the coax-feed where it is connected to the split. At the receive side, use lumped port from 1mm width metal to another metal at the bottom, assuming you are using this as GND.

Hope this makes sense based on my understanding of your problem.