Defining arbitrary objective function+Matlab

Hello,
I have a question about the Bilinear.m example in https://www.gurobi.com/documentation/9.0/examples/bilinear_m.html.

In this example, the objective is to maximize the variable x.
But how can I change it to have an arbitrary function to maximize or minimize?
For example, I want to minimize the Euclidean norm of some of the variables.

Sorry if it is a basic question!
Thank you in advance.