with jQuery :
html :
<form method="POST" name="myform" action="index.php?action=contact_agent&agent_id=" onsubmit="SetData()">
<select name="agent" id="agent">
<option value="1">Agent Homer</option>
<option value="2">Agent Lenny</option>
<option value="3">Agent Carl</option>
</select>
</form>
jQuery :
$('form').submit(function(){
$(this).attr('action',$(this).attr('action')+$('#agent').val());
$(this).submit();
});
javascript :
function SetData(){
var select = document.getElementById('agent');
var agent_id = select.options[select.selectedIndex].value;
document.myform.action = "index.php?action=contact_agent&agent_id="+agent_id ; # or .getAttribute('action')
myform.submit();
}