Adding Sub category under category

Hi,
I have a scenario where I have a dropdowns for category and sub category. I want to add the sub categoy under the respective sub category from the front end. I have done the coede for category section. Please help me with to add the sub-category under the respective category. 
Please see the code  so far which I did:-
<div> <asp:TextBox ID="txtCategoryAdding" runat="server"></asp:TextBox> <asp:TextBox ID="txtDescription" runat="server" TextMode="MultiLine"></asp:TextBox><br /> <br /> <asp:Button ID="btnAdd" Text="Add Category" Width="100" runat="server" OnClick="btnAdd_Click" /> </div> protected void btnAdd_Click(object sender, EventArgs e) { using (SqlConnection con = new SqlCo ...