[CST]optimization to enhance the bandwidth

As is proposed in some previous studies, i use an added U-slot on an octagonal patch, expecting to enhance the bandwidth.
Actually, there are at least 3 important parameters:
1) the feedpoint of the coaxial line
2) the length of the U-slot
3) the width of the U-slot
I tried to apply the optimizer, yet i don't know how to define the "user defined goal function"
Could someone give me some suggestions? Thank you very much!