cannot retrieve the value from modal popup extender control?

Hi sir,
    Tell me the steps to retrieve the value from modal popup extender control. I tried the below code to get the value from the popup, but i cant able to retrieve the value, Kindly give me the solution for this problem.


Default.aspx:
************

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="asp" %>





   


   

   

    
       
       
        
       
      
                    DynamicServicePath="" Enabled="True" TargetControlID="Button1" DropShadow="true"  PopupControlID="Panel1" OkControlID="btn_ok" CancelControlID="btn_cancel">
       
      
       

       

       
       
       

       

        
       

       

    
       
       
       
           
                Yes
                No
           
           

           

           
             
       
       
       

       
         

         
       
    
   

   

Default.aspx.cs:
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 protected void btn_ok_Click(object sender, EventArgs e)
    {
        TextBox1.Text = RadioButtonList1.SelectedItem.Text.ToString();
    }