Add data from control on page to a HyperLinkField in a column of a Gridview

How do I add data from control on page to a querystring of a HyperLinkField in a column of a Gridview?
I tried this but it does not work. DataNavigateUrlFormatString="~/Application/Sales/Managers/InventoryDetail.aspx?LocationsID={0}&StockType=" & StockTypeDropDownListBox.SelectedValue
 FYI - StockTypeDropDownListBox is not in the gridview.
<asp:HyperLinkField DataNavigateUrlFields="LocationsID" DataNavigateUrlFormatString="~/Application/Sales/Managers/InventoryDetail.aspx?LocationsID={0}&StockType=" DataTextField="TotalCars" HeaderText="TotalCars" SortExpression="TotalCars" DataTextFormatString="{0:n0}"> <HeaderStyle Width="100px" HorizontalAlign="Right" /> <ItemStyle HorizontalAlign="Right" CssClass="DailyNCD" /> </asp:HyperLinkField> Thank ...