How to do antenna optimization using HFSS?

i dont think optimization is much useful
u can try parametric sweep

Use tuning if you wish to see differences. You have add some variables such as feed points or dimensions. And you can use theese variables on tuning with sweep. after tuning, you can see the variables effects on design. You can quickly see results with quick report. Depend on variables and steps tuning could take some time. (1 hour, 1 day or 1 week)

Good luck.

try usin DGS and DMS for optimizing microstrip antennas