Ok.
Backup your form's file.
Just change the method of your form with GET instead of POST. Insert a target="_blank".
Then try to submit.
You will get a new page with the query you need.
Suppose is something like http://www.myurl.com/login.php?username=bad76&password=unknow
ok?
You now can use this link directly in the page (but this give access at everyone...) or ask to your customer for username and password.
To do this you can use a javascript like this to ask the user and password when the link is clicked:
function askpwd()
{
myusr=prompt("Please insert a username","");
mypwd=prompt("Please insert a password","");
document.location="http://www.myurl.com/login.php?username="+myusr+"&password="+mypwd
}
... // after in the page...
<a href="#" onclick="askpwd(); return false;">go inside </a>
I hope this helps...
bye bye