you can simply use your own code but add name for the select tag
<form method="POST" action="index.php?action=contact_agent&agent_id=">
<select name="agent_id">
<option value="1">Agent Homer</option>
<option value="2">Agent Lenny</option>
<option value="3">Agent Carl</option>
</select>
then you can access it like this
String agent=request.getparameter("agent_id");