How to show modalpopupextender in textchange event

<form id="form1" runat="server">
   <div style="min-height: 700px;">
       <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
       </asp:ToolkitScriptManager>
       <asp:Button ID="Button1" runat="server" Text="Click here to show the modal" />
       <asp:TextBox ID="TextBox1" runat="server" AutoPostBack="true"
           ontextchanged="TextBox1_TextChanged"></asp:TextBox>
       <asp:ModalPopupExtender ID="ModalPopupExtender1" BackgroundCssClass="ModalPopupBG"
           runat="server" CancelControlID="btnCancel" OkControlID="btnOkay" TargetControlID="Button1"
           PopupControlID="Panel1" PopupDragHandleControlID="PopupHeader" >
       </asp:ModalPopupExtender>
       <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
       <ContentTemplate>
       <div id="Panel1" style="display: none;" class="popupConfirmation">
           <div class="popup_Container">
               <div class="popup_Titlebar" id="PopupHeader">
                   <div class="TitlebarLeft">Popup Header</div>
                   <div class="TitlebarRight"></div>
               </div>
               <div class="popup_Body">
                   <asp:Repeater ID="Rptr_popup" runat="server">
                   <ItemTemplate>
                  <%-- <asp:LinkButton ID="lnkbtn_Contperson" runat="server" Text='<%# Eval("contname")%>'></asp:LinkButton>--%>
                   <asp:RadioButton ID="RadioButton1" runat="server"  Text='<%# Eval("contname")%>' TextAlign="Left"/>
                   </ItemTemplate>
                   </asp:Repeater>
               </div>
               <div class="popup_Buttons">
                   <input id="btnOkay" value="Done" type="button" />
                   <input id="btnCancel" value="Cancel" type="button" />
               </div>
           </div>
       </div>
       </ContentTemplate>
       <Triggers>
       <asp:AsyncPostBackTrigger ControlID="TextBox1" />
       </Triggers>
       </asp:UpdatePanel>
   </div>
   </form>

protected void TextBox1_TextChanged(object sender, EventArgs e)
        {

            ModalPopupExtender1.Show();
        }