[phil_]

ok, I worked it out. You can just do it simply with javascript and an onchange event.

In javascript functions:

function set_parameter(){
document.formname.param_to_pass.value = document.formname.custom_drop_down.options[document.formname.custom_drop_down.selectedIndex].value;
}

Then in your form:

<select name="custom_drop_down" onchange="set_parameter()">
<option selected value="one">Select your option...</option>
<option value="two">Option 1</option>
<option value="three">Option 2</option>
</select>

<input type="hidden" name="param_to_pass" value="one">

now on changing the drop down, it updates the value of param_to_pass to whatever the selected option value is.

Philip.