Ajax tool kit in dynamic html control

<%@ Register
    Assembly="AjaxControlToolkit"
    Namespace="AjaxControlToolkit"
    TagPrefix="ajaxToolkit"%>

Add AJAX Script Manager to the page:
<asp:ScriptManager ID="ScriptManager1"runat="server"/>

Add DynamicPopulateExtender control to the page:

  <ajaxToolkit:DynamicPopulateExtender
          ID="dp"BehaviorID="dp1"runat="server"
          TargetControlID="form1$Label1"
          ClearContentsDuringUpdate="true"
          ServiceMethod="GetHtml" />