function add_sf_instance_url( $settings ) {

    $settings['sf_instance_url'] = array(
            'title'   => 'Instance URL',
            'desc'    => '',
            'std'     => '',
            'type'    => 'text',
            'section' => 'setup'
        );

    return $settings;

}

add_filter('wpf_configure_settings', 'add_sf_instance_url');