redirection to a page based on confirmation

Try this

ClientScriptManager CSM = Page.ClientScript;

            string strconfirm = "<script>if(window.confirm('Are you sure?')){window.location.href='Default2.aspx'}</script>";
            CSM.RegisterClientScriptBlock(this.GetType(), "Confirm", strconfirm, false);

This code writes to output instructions for browser to show alert message user. But this output will never be sent to the client, because next line instructs your web application to send this client response with code 302 and address to which the user must be redirected instead.
You should use Javascript to perform redirect from the client. The code would look loke the following:

Response.Write("<script>alert('my message'); ");
Response.Write("window.location='[new address]'</script>");

In the linkbutton, you can used the onclientclick event is used to show the confirm message and onclick event used to redirect the page

<asp:LinkButton  ID="btn_Cancel"  runat="server" Text="Cancel"  OnClientClick="return confirm('Are you sure want to cancel this Request?')"   />

You can rewrite the confirm text as alert message.

try in the following way this must work :

ClientScript.RegisterStartupScript(typeof(Page), "MessagePopUp", "<script
language='JavaScript'>alert(ABCDEFGH');</script>");

Response.Redirect("Default.aspx");

and u to redirect in javascript also :

alert ('ABCDEFGH'); window.location.href = 'newpage.aspx';

<code>ClientScriptManager CSM = Page.ClientScript;
 
            string strconfirm = "<script>if(window.confirm('Are you sure?')){window.location.href='Default2.aspx'}</script>";
            CSM.RegisterClientScriptBlock(this.GetType(), "Confirm", strconfirm, false);</code>