Enrolling into graduate courses as an undergrad

Does anyone know what the process is for taking graduate level courses as an undergraduate, particularly in the bioengineering (BENG) and biology (BGGN) departments?

I’m looking to take 1-2 courses in either department; the courses I’m looking at say that undergrads can take the course “with consent of instructor.” Does anyone know what the process is for getting instructor’s approval? I don’t see a petition function on WebReg like they have for undergrad courses; is there another process for this?

Thanks.