How to open a popup window

Hello,

i using the WordPress Menu feature and wanted to display a link to my customer Account area. So i thought, i add a link to the menu and if the user clicking on it, a popup window opens where he can sign in to his account (not wordpress account).

i added to header.php this popup window code

<script language="JavaScript" type="text/javascript">
function popup(URL) {
w = window.open(URL, "", "toolbar=0,scrollbars=1,location=0,statusbar=0,menubar=0,resizable=0,width=960,height=800,left = 480,top = 200");
}
</script>

and added to the menu entry this url

javascript:popup('mylinktoaccount.php')

but that didn’t go that well.

Is there an other way, to get it done and working?

Thanks in advance