how to get primary key from dropdownlist using sqldatasource and save into another table

hi i have a dropdownlist with subcat name and a product info form
basicaly i wants when user select subcat name than he enter product information so subcat selected name primary key save in product table <table>               <tr>           <td>               <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1" DataTextField="companyName" DataValueField="brandid"></asp:DropDownList>               <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:OnyxTheLaptopStoreConnectionString %>&quo ...