Using the reference: Server Side (Code Behind) Yes No Confirmation Message Box in ASP.Net
<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>
<asp:TextBox runat="server" AutoPostBack = "true" OnTextChanged = "OnTextChanged" onchange = "Confirm()" />
protected void OnTextChanged(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);
}
}