How to save the URL of anonymous user on a webform?
So you can achieve this using form alter, 

create a text field namely 'refer_url' for saving referring url, then using following snippet in your module you can save the referring url. You can hide this field in display using css or form alter.

function module_name_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if ($form_id == 'webform_client_form_27') { 
    $form['submitted']['refer_url']['#default_value'] = $_SERVER['HTTP_REFERER'];
    }
}