Here is my Code

protected void Page_Load(System.Object sender, System.EventArgs e)
        {
            try
            {
                if (!Page.IsPostBack)
                {
                    if (((string)Settings["template"] != null) && ((string)Settings["template"] != ""))
                        strTemplate = (string)Settings["template"];
                    else
                        strTemplate = Localization.GetString("Template.Text", LocalResourceFile);

                    testController objtests = new testController();

                    if (ClientAPI.BrowserSupportsFunctionality(ClientAPI.ClientFunctionality.XMLHTTP) && ClientAPI.BrowserSupportsFunctionality(ClientAPI.ClientFunctionality.XML))
                    {
                        ClientAPI.RegisterClientReference(this.Page, ClientAPI.ClientNamespaceReferences.dnn_xml);
                        ClientAPI.RegisterClientReference(this.Page, ClientAPI.ClientNamespaceReferences.dnn_xmlhttp);
                    }

                    this.Button1.Attributes.Add("onclick", ClientAPI.GetCallbackEventReference(this, "dnn.dom.getById('" + testLBL.ClientID + "').value", "successFunc", "'" + Button1.ClientID + "'", "errorFunc"));

                    if (this.Page.IsClientScriptBlockRegistered("helloajax.js") == false)                      
                        this.Page.RegisterClientScriptBlock("ajax.js",
                            "<script language=\"javascript\" src=" + "\"" + this.ModulePath + "ajax.js" + "\"></script>");
                }
            }
            catch (Exception exc) //Module failed to load
            {
                Exceptions.ProcessModuleLoadException(this, exc);
            }

        }

        public string RaiseClientAPICallbackEvent(string eventArgument)
        {
            return "HELLO: " + eventArgument;
        }

<%@ Control language="C#" Inherits="YourCompany.Modules.test.Viewtest" CodeFile="Viewtest.ascx.cs" AutoEventWireup="true"%>
<%@ Register TagPrefix="dnn" TagName="Audit" Src="~/controls/ModuleAuditControl.ascx" %>
<asp:Label ID="testLBL" runat="server" Text="This is a Test Label"></asp:Label>
<br>
<input id="Button1" type="button" value="button" name="Button1" runat="server"/><br>