As you posted here I asume that you are not using a server side language... In that case I can think of a way but the ID will not be completely unique always. This script adds the date to the subject of the mail, it uses miliseconds so it isnt very likely to have the same ID twice, though its possible

.
<form name="form" action="mailto: " method="post" onsubmit="sendformDate();">
<input type="hidden" name="ID_No." id="hid"/>
<input type="submit" />
</form>
<script type="text/javascript">
function sendformDate(){
var codeID=new Date();
document.getElementById('hid').value=codeID.getTime();
}
</script>