Embedding a cookie via PHP in Contact Form 7

Hi Guys,

I’ve been asked to embed the following into our contact forms:
<?php
$value = $_GET[‘refid’];
setcookie(“refid”, $value, time() + (86400 * 30), ‘/’, ‘.domain.co.uk’);
?>

How would I go about getting this into Contact Form 7 as a hidden field?

Thanks in advanced!

Dan