Show Popup Without Ajax At Checkout

Hi i am developing my payment gateway. i have to show popup during place order. if customer press complete order. popup have to open line,

$res = Registry::get('view')->fetch('addons/my_gateway/views/auth/confirm.tpl');
Registry::get('ajax')->assignHtml($id, $res);

i can show popup with ajax. but checkout form not ajax.

Registry::get('display')->... 

i try with display then going new url with displayed tpl,

basically i have to open popup

at here

if (defined('PAYMENT_NOTIFICATION')) {
}else{
if(...)
open_popup
exit;
}

any one can help me ?