Insurance and plastic surgery

I know that this question has been asked many times but I cannot find anything on the website. I am planning on having a tummy tuck in the future and I have BCBS Federal. They have informed me that they will pay for my surgery if I can prove that it is medically necessary. My regular doctor told me that he would write the letter and say whatever I need him to say to get my surgery. Well, I don't know what it needs to say. I just need a sample letter or any info that I can get. With my insurance, you have to have the surgery first THEN it gets sent to insurance. They decide after you have the procedure so it is a gamble. But they paid for my bypass surgery so a lady at a plastic surgeon's office says that I stand a very good chance of them paying for a tummy tuck. I just want to make sure all of this is taken care of before I have the surgery so I don't get stuck with the bill. ANY info will help! Thanks!