Attaching Arguments With Window.open?

I'm a PHP developer that needs to user javascript to open a window prior to a _POST call.

PHP Code:
<title>test</title>
<SCRIPT language="JavaScript">
function newwindow()
{
var myForm = document.header;
var txtValue = myForm.txtUsername.value;
var txtFileName = "lost_logon.php";
[Code]....

I need the window.open to open a new window with arguments attached. For example is the username is bigbob the call should be lost_login.php&user=bigbob. How do I pass the argument?