How to get the referring url?

Hi,
I inserted the following code into functions.php to pass the referring url in the form:

<?php 
function populate_referral_url( $form ){
    // Grab URL from HTTP Server Var and put it into a variable
    $refurl = $_SERVER['HTTP_REFERER'];
 
    // Return that value to the form
    return esc_url_raw($refurl);
}
?>

Now I would like to use a hidden field to get the referring url transmitted by sending the form. But what do I have to put in the hidden field exactly. I′ve tried everything, but I′m not able to do it.

(Value): Is it {GET:refurl}? or {GET:$refurl}? or other code? Nothing is working. thank you!!