The simplest way to do it, see below.
Don't forget to set ValidateRequest=false.
For .NET 4.0 it also needed to add into your web.config file the following line
<system.web>
<httpRuntime requestValidationMode="2.0" />
<compilation debug="true" targetFramework="4.0" />
</system.web>
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="CodeProjectWeb.WebForm1"
ValidateRequest="false" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<table>
<tr>
<td>
<asp:Button ID="btnClick" runat="server" Text="show" OnClick="btnClick_Click" /><br />
<asp:TextBox runat="server" ID="txtText" TextMode="MultiLine" Rows="5" Columns="50"></asp:TextBox>
</td>
<td valign="top">
<%if (this.IsPostBack) { Response.Write(Server.HtmlDecode((txtText.Text))); } %>
</td>
</tr>
</table>
</div>
</form>
</body>
</html>
Good luck.