how to Called javascript:confirm() alert from codebehind

string msg1;
       msg1 = "<script language='javascript'>alert('Are You Sure You Want To Delete Madhuri s Data !');</script>";
       if (!ClientScript.IsStartupScriptRegistered("error"))
           {
               RegisterStartupScript("error", msg1);
           }

string msg1;
        msg1 = "<script language='javascript'>confirm('are You sure you want to delete Madhuri s data ...!');</script>";
        if (!ClientScript.IsStartupScriptRegistered("error"))
        {
            RegisterStartupScript("error", msg1);
        }

function Confirm() {
   var confirm_value = document.createElement("INPUT");
   confirm_value.type = "hidden";
   confirm_value.name = "confirm_value";
   if (confirm("Do you Want to View DashBoard?")) {
   confirm_value.value = "Yes";
   } else {
   confirm_value.value = "No";
   }
   document.forms[0].appendChild(confirm_value);
   }

<asp:imagebutton id="btnSwitch2" runat="server" xmlns:asp="#unknown">
                       ImageUrl="~/Images/dptch.jpg" OnClick="OnConfirm" 
                       OnClientClick="javascript:Confirm();" style="width: 20px" Width="20px" 
                       Height="21px" ImageAlign="AbsMiddle" Visible="False" />


</asp:imagebutton>

 Protected Sub OnConfirm(ByVal sender As Object, ByVal e As EventArgs)
      Dim confirmValue As String = Request.Form("confirm_value")
        If confirmValue = "Yes" Then
---Do what you want when Yes
        Else
 ---Do what you want when No
        End If
    End Sub

protected void OnConfirm(object sender, EventArgs e)
{
	string confirmValue = Request.Form("confirm_value");

	if (confirmValue == "Yes") {
----Do what You want when Yes
	}
	 else {
----Do what You want when  No
	}
}