Saya akan berikan fungsi javascript untuk popup di HTML
function pop_up() //fungsi popup
{
var width = 800;
var height = 400;
var left = (screen.width - width)/2;
var top = (screen.height - height)/2;
var params = 'width='+width+', height='+height+',scrollbars=yes';
params += ', top='+top+', left='+left;
window.open('form_popup.php','Form Popup',params);
};