Gastric Sleeve Surgery

I have a consultation for gastric sleeve surgery. I know BCBSIL covers it, but my specific policy only covers it if medically necessary. Can you tell me specifically what the process is to go about getting it covered? Do I need to have the surgeon at the consultation submit a predetermination form? Do I need any other kinds of forms? Please tell me the steps I need to follow to get approved. I already know I medically qualify for the surgery, I just need to know how to let insurance know...? Thank you.