Display Server Side Yes No Confirmation Box on DropDownList SelectedIndexChanged in ASP.Net

Hi,

 I am trying to use Yes No Confirmation Message Box but the code below here which i found here uses a button to raise the confirmation message box but what i would like to use is a DropDown.  How can i modify the code and i use drop-down box instead of using a button?

here is the javascript code: 

 <script type = "text/javascript">
         function Confirm() {
             var confirm_value = document.createElement("INPUT");
             confirm_value.type = "hidden";
             confirm_value.name = "confirm_value";
             if (confirm("Do you want to save data?")) {
                 confirm_value.value = "Yes";
             } else {
                  confirm_value.value = "No";
             }
             document.forms[0].appendChild(confirm_value);
         }
    </script>

here is the aspx:

<div>
    <asp:Button ID="btnConfirm" runat="server" OnClick = "OnConfirm" Text = "Raise Confirm" OnClientClick = "Confirm()"/>
    </div>

code behind:

  public void OnConfirm(object sender, EventArgs e)
    {
        string confirmValue = Request.Form["confirm_value"];
        if (confirmValue == "Yes")
        {
            this.Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked YES!')", true);
        }
        else
        {
            this.Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked NO!')", true);
        }
    }

but here is what i would like to use is drop-down instead of the button:

        <div>
             <asp:DropDownList ID="ddl" Width="300px" runat="server" AppendDataBoundItems="true" OnSelectedIndexChanged="ddl_SelectedIndexChanged">
                        <asp:ListItem Text="test1" Value="test1"></asp:ListItem>
                        <asp:ListItem Text="test2" Value="test2"></asp:ListItem>
                        <asp:ListItem Text="test3" Value="test3"></asp:ListItem>
                    </asp:DropDownList>

        </div>

Please help.  thanks