How can I get the fitness function for GA from a Simulink system model?

Hi, I'm working on a Three area LFC system with PI controller, I've already modeling the system in simulink. And I want to use GA to optimize the PI controller in the system, I tried to use the toolbox, but I dont have any fitness function, and I can't calculated it. Now, I have a ITAE with a yout model to workspace(as the picture shown). But I don't know how to write the GA code to analyze the signal. Can anyone help me please?

Thank you so much!