You can avoid filling out the form, but you aren't anonymous. Your post is logged with a timestamp and your IP address. One subpoena to your ISP asking for the identity of the person logged onto that address at that date and time, and there ya go!
If you really need anonymitity, I expect library computers might be a good place from which to submit an item. Personally, I'm not worried about the Jihadists getting the FBI submissions, tracking them backwards, and taking retribution.