Why this simple example of ajaxToolkit:DynamicPopulateExtender does not work?!!

<form>
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="true" />
 
<ajaxToolkit:DynamicPopulateExtender ID="dpe1" runat="server"
     ClearContentsDuringUpdate="true"
     TargetControlID="txtContent" ServiceMethod="HelloWorld" ServicePath="OpenTextFile.asmx.vb"     />
     
<asp:Panel ID="pnlEditor" runat="server" >
   <input type="button" onclick="$find('dpe1').populate();" ID="Button1" value="Button" />
</asp:Panel>
        <asp:Label ID="txtContent" runat="server" Text="Label"></asp:Label>
</form>
 
And ere is the service file:
 
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.ComponentModel
Imports System.Web.Script.Services
 
<System.Web.Services.WebService(Namespace:="http://tempuri.org/")> _
<System.Web.Services.WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
<ToolboxItem(False)> _
Public Class OpenTextFile
    Inherits System.Web.Services.WebService
 
    <WebMethod()> _
    Public Function HelloWorld() As String
       Return "Hello World"
    End Function
 
End Class

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