Web Forms :: Display Server Side Yes No Confirmation Box On DropDownList SelectedIndex Changed

 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";

[code]....

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>