click on link button and it's display lable that select this first

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ASP.NET

hello

When user want to add product in cart and click on add to cart link button than it's display select size first so how i can do it

my code for add to cart:

<asp:LinkButton ID="linkAddtocart" runat="server"  oncommand="BuyNow_Command"
               CommandArgument='<%#Eval("Product_ID") %>'>Add To Cart</asp:LinkButton>

protected void BuyNow_Command(object sender, CommandEventArgs e)
   {

       string str = "";
       if (Session["ProductID"] == null)
       {
           str = e.CommandArgument.ToString();
           Session["ProductID"] = str + "|";
       }
       else
       {
           str = Session["ProductID"].ToString();
           str = str + e.CommandArgument.ToString() + "|";
           Session["ProductID"] = str;
       }

       Label1.Text = str;

   }

and lable and it's value of sizes:

<asp:Label
               ID="lblSize"
               runat="server"
               Text="Select Size :"></asp:Label>

               <asp:RadioButton ID="RadioButton1" GroupName="Size" runat="server" Text="Small"
               oncheckedchanged="RadioButton1_CheckedChanged" />
               <asp:RadioButton ID="RadioButton2" GroupName="Size" runat="server"
               Text="Medium" oncheckedchanged="RadioButton2_CheckedChanged" />
               <asp:RadioButton ID="RadioButton3" GroupName="Size" runat="server"
                Text="Large" oncheckedchanged="RadioButton3_CheckedChanged" />
               <asp:RadioButton ID="RadioButton4" GroupName="Size" runat="server"
                Text="XL" oncheckedchanged="RadioButton4_CheckedChanged" />
               <asp:RadioButton ID="RadioButton5" GroupName="Size" runat="server"
                Text="XXL" oncheckedchanged="RadioButton5_CheckedChanged" />
               <asp:RadioButton ID="RadioButton6" GroupName="Size" runat="server"
                Text="XS" oncheckedchanged="RadioButton6_CheckedChanged" />