Javascript prompt and submit

i want to get a value from JavaScript prompt and pass it to input for finally submit it

<form id="createdirForm"><input type="text" name="" id="createdir"/><form>
        <script type="text/javascript">
        function createdir(){
            var newdirname;
            newdirname = prompt('Please input the directory name:', '');
            if (!newdirname) return;
            $('createdir').newdirname.value = newdirname;
            $('createdirForm').submit();
        }
        </script>