Código HTML:
<form name="forma1" method="post" action="forma2.html">
<select name="menu" size="3" onChange="document.forms['forma1'].submit()">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<input type="submit" name="Enviar">
</form> Espero que sea lo que buscas. Saludos