codice:
<?xml version="1.0" encoding="iso-8859-1"?>

  <!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" xml:lang="en" lang="en">
    <head>
      <title>Code Example by DYNAMIC+</title>

      <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
      <meta http-equiv="Content-Language" content="en-us" />

      <script type="text/javascript">
      <!--

        function scrivi( )
        {
          document.forms['form1'].elements['testo'].value = "Andrea";
        }
       
      //-->
      </script>

    </head>
    <body>

      <div>

        <form name="form1" method="post" action="prova.html" onsubmit="scrivi( );">
          <label> 
            <input type="text" name="testo" value="" /> 
            <input type="submit" name="Submit" value="Invia" />
          </label>
        </form>

      </div>

    </body>
  </html>
ps: l'indentazione è importante!