Per inviare i dati gli elementi debbono avere un name

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Untitled Page</title>

    <script type="text/javascript">
        window.onload = aggiornaCampo;

        function aggiornaCampo() 
        {
            document.getElementById('abilitato').value='xxxxx';
        }

    </script>

</head>
<body>
    <form action="a.aspx" method="post">
        <input type="hidden" value="" id="abilitato" name="abilitato" />
        <input id="Submit1" type="submit" value="submit" />
    </form>
    
    
</body>
</html>