Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>User Manager</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script language="JavaScript" type="text/JavaScript">
var site= '.mysite.com';
window.onload=function(){
    var frm=document.getElementsByTagName('form')[0];
    frm.elements[1].onclick=function(){
        var sub= frm.elements[0].value;
        location.href= 'http://www.'+sub+site;
    }
}
</script>
</head>
<body>
<form action="post.php" method="post" name="frm">
<input name="sub" type="text">
<input name="go" type="button" value="go">
</form>
</body>
</html>