Cannot Set Referrer or First_Visited_URL via the API

Hello,

I'm working on a custom form that needs to do several things on our website, one of which is to add a lead into Zoho CRM. We used to use a Zoho Webform for this, but the number of things we now have going on make that impossible.

I'm trying to set either the 'referrer' field or 'first_visited_url' field when a I add the lead to Zoho, but neither seem to work nor do they return any errors. The lead gets added with the rest of the fields I have specified, and those fields blank.

The date I'm posting to the API is derived from this:

$data_string = json_encode( array(    "data" => array([    "Owner"                    => "####",
                                                                    "First_Name"            => $first_name,
                                                                    "Last_Name"                => $last_name,
                                                                    "Company"                => $company,
                                                                    "Email"                    => $email,
                                                                    "Phone"                    => $phone,
                                                                    "Country"                => $country,
                                                                    "Lead_Start_Date"        => date("Y-m-d"),
                                                                     "Referrer"                => "https://www.google.com/"
                                                                ])));

Any thoughts or ideas on how I can get this data in their would be greatly appreciated. It used to flow in from salesIQ, but I've gone through a dozen roads and that doesn't seem to want to integrate smoothly with the API methods like it did with the Webform.