Convex or Concave edge Horn antenna in HFSS

Usually, we have straight edge horn antenna's.But I want to simulate a concave edge horn antenna in HFSS. I have simulated the usual straight edge horn but I don't know how to make the edges concave.I have attached the picture of what I want to simulate.Please help me.Thanks in advance.

I think it can be done using arc but I don't know how to do that.