You don't give up do you? mailto: is obsolete - you should use PHP to transmit your form.
But to answer your question:-
Code:
<select id = "interest" onchange = "getVal()">
<option value = "0">Select your interest...</option>
<option value = "Drinking">Drinking</option>
<option value = "Gambling">Gambling</option>
<option value = "Womanising">Womanising</option>
</select>
<script type = "text/javascript">
function getVal() {
var val = document.getElementById("interest").value;
var emadd = "me@mydomain.com";
if (val !=0) {
emadd = emadd +"?subject=" + val;
alert (emadd);
}
}
</script>
You cannot format the form data in any way. And, for the third time, modern browsers no longer accept mailto: as a form action - they simply open the email program (if any) and ignore the form.