javascript on ajax's extender control in asp.net

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Javascript

Ajax

ASP.NET4

can i apply javascript on that control of asp.net that have ajax extender on it ??
because when i using javascript on textbox having watermark extender of ajax then javascript code coudnt be work ??

javascript::

<script type ="text/javascript" language = "javascript" >
        function validate() {
            if (document.getElementById("<%=txtName.ClientID%>").value == "") {
                alert("please enter your name here ");
                document.getElementById("<%=txtName.ClientID%>").focus();
                return false;
            }
            
        }
</script>

<asp:TextBox ID="txtName" runat="server" Width="100px" ForeColor="#999999">Last Name
<asp:TextBoxWatermarkExtender ID="txtName_TextBoxWatermarkExtender" 
                                        runat="server" Enabled="True" TargetControlID="TextBox2" 
                                        WatermarkText="Last Name">
<asp:Button ID="Button1" runat="server" Text="Button" />

CodeBehind::

Protected Sub Button1_Click(sender As Object, e As System.EventArgs) Handles Button1.Click
       Button1.Attributes.Add("onclick", "return validate()")
   End Sub

here are the code which i tried but not worked...