function populate_posts($form){

	foreach($form['fields'] as &$field){

		if($field['defaultValue'] != '{referer}')
			continue;

        //Check referrer field, if exists and HTTP referrer not set, try to fill it manually
        if($field['defaultValue'] == '{referer}' && ! isset($_SERVER['HTTP_REFERER']))
        {
            $field['defaultValue'] = _getReferrerUrl();
            continue;
        }
	}

	return $form;
}

function _getReferrerUrl()
{
    $url = 'Referrer niet beschikbaar';

    if(isset($_GET['occ_id']))
        $url = get_option('home') . '/occasion/' . $_GET['occ_id'];

    if(isset($_GET['post_id']))
        $url = get_permalink($_GET['post_id']);

    return $url;
}

//Add these for each form, where the last number is the form id
add_filter('gform_pre_render_1', 'populate_posts');
add_action("gform_admin_pre_render_1", "populate_posts");