postback example in asp.net

// On the button control, inside the event
// Check if indicating to continue or intercept, open the popup,
// and define the callback with the "sender" (the button itself)
if (!(bool)Session["IsAllowed"])
{
        // Open popup to enter the password, and define a callback
        Page.RegisterStartupScript(
                "openwindow", "<script>window.open('http://myurl.aspx'" +
                ",'title');" +
                "myPage_Callback = function(){" +
                Page.GetPostBackEventReference((System.Web.UI.Control)sender,"") +
                "};" +
                "</script>"
        );

        return;
}
// On popup window, on event of the submit button 
// Calls the __doPostBack callback js function on the opener and closes the popup.
Session["IsAllowed"] = true;
Page.RegisterStartupScript(
        "closewindow", "<script>window.opener.window.myPage_Callback();" +
        "window.close();" +
        "</script>"
);