Ask HN: How to get referrer source of iframe parent page?

I'm looking to place to iframe a form into a page, and when someone lands on that page and enters their information into the form, I want to grab the referrer source.

For example: I embed the iframe onto website.com/contact. The URL of that page gets posted to Facebook. Someone clicks the link, lands on the page, and enters their information into form. I then want to be able to say, 1 referral came from Facebook.

I tried using $_SERVER['HTTP_REFERER'], but this just returns the parent page URL of where the iframe form is embedded, which I don't want. I want the actual URL of the referral to the parent page (in the example above, it would be Facebook). Is there anyway to grab this information?