Importing AdWords Conversions from Salesforce.com via CF7 & GCLID value

I think the issue I am facing is being able to run this script within the form to get the GCLID cookie and then pass it to CF7 via a dynamic hidden field:

<script>
        window.onload = function getGclid() {
             document.getElementById("xxxx").value = (name = new
        RegExp('(?:^|;\\s*)gclid=([^;]*)').exec(document.cookie)) ?
        name.split(",")[1] : ""; }
        </script>

xxxx = form field ID (for example “gclid_field”)

For anyone wondering how to do this I think I have resolved the issue. Thanks to the help from https://www.sean-barton.co.uk/2014/04/contact-form-7-place-post-server-cookie-session-variables-fields/#.VkstpXbhCes

function sb_cf7_cookie($atts){
    extract(shortcode_atts(array(
        'key' => -1,
    ), $atts));

    if($key == -1) return '';
    $val = '';

    if( isset( $_COOKIE[$key] ) ){
        $val = $_COOKIE[$key];
    }

    return $val;
}
add_shortcode('SB_CF7_COOKIE', 'sb_cf7_cookie');

Once this has been added to functions.php you can call it using dyanmic text extension, as followed:

[dynamichidden gclid_field "SB_CF7_COOKIE key='__utmz'"]

I hope this helps others!