Please help me modify my aspx page markup code to show a Modal dialog message

I need to show a modal dialog message of "You can not forward the letter because the required answering period already finished. You only can reply". The message have "OK" button that when clicked, the user is redirected to "~\Pending\PendingsPostings.aspx" page.

The page ForwardPosting.aspx which once its Page_Load event is executed, the modal dialog message should shown. When the message is shown, the page "MainFields" Div should become with fully grey style.
The ForwardPosting.aspx page is a content page. Its markup code is as following:
<%@ Page Title="" Language="VB" MasterPageFile="~/MasterPages/MasterPage.master" AutoEventWireup="false" CodeFile="ForwardPosting.aspx.vb" Inherits="NewPosting_ForwardPosting" MaintainScrollPositionOnPostback="true" %> <asp:Content ID="Content1" ContentPlace ...