Apertura pagina a scelta tramite form

<form method="POST" action="login.php" id="formL">
<input type="text" name="user" id="user" placeholder="Login" />
<input type="password" name="pass" id="user" placeholder="Password" />
<br />
<input type="checkbox" id="popupSelector" /> Apri in una nuova finestra (popup)<br />
<input type="submit" value="Invia" />
</form>

<script type="text/javascript">
document.getElementById('formL').onsubmit = function(){
if(document.getElementById("popupSelector").checked==true){
var wi = screen.availWidth - 10;
var au = screen.availHeight - 30;
{window.open ('','game', "width=" + wi + ",height=" + au + ",top=0,left=0,location=no,menubar=no,resizable=yes,scrollbar=yes");}
document.getElementById("formL").target='game';
}
}
</script>