That's because your adding items at client-side and in server-side the drop-down is empty. You can have a hidden input:
<input type="hidden" id="selectedValue" runat="server" />
And change its value when the value of the drop-down changes:
$('#MyDropdown').change(function () {
$('#selectedValue').val($(this).val());
}
At the server you read the value of your hidden input.