@Deanna I think I know what you're looking for now. What I'd do is add some jQuery/JavaScript to the page the form is on. Create an event for the dropdown to populate the textbox. As David mentioned and in my tests too, it only populates the first item. So a simple solution is to use some client side code.
Sample rendered HTML
<select id="stores">
<option value="store1">Store 1</option>
<option value="store2">Store 2</option>
<option value="store3">Store 3</option>
<option value="store4">Store 4</option>
</select>
<input name="storenum" type="text" id="storenum" />
jQuery
$("#stores").change(function () {
$("#storenum").val($(this).val());
});
This will allow you to use the standard form functionality and simply add 3 simple lines of javascript to get your functionality to work as expected.