Javascript to set hidden form value on drop down change

If you have HTML like this, for example:

<select id='myselect'>
    <option value="1">A</option>
    <option value="2">B</option>
    <option value="3">C</option>
    <option value="4">D</option>
</select>
<input type="hidden" id='myhidden' value="">

All you have to do is bind a function to the change event of the select, and do what you need there:

<script type="text/javascript">
$(function() {
    $('#myselect').change(function() {
        // if changed to, for example, the last option, then
        // $(this).find('option:selected').text() == D
        // $(this).val() == 4
        // get whatever value you want into a variable
        var x = $(this).val();
        // and update the hidden input's value
        $('#myhidden').val(x);
    });
});
</script>

All things considered, if you’re going to be doing a lot of jQuery programming, always have the documentation open. It is very easy to find what you need there if you give it a chance.