how to use water mark extender control in ajax?

<asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:TextBoxWatermarkExtender ID="TextBox1_TextBoxWatermarkExtender" 
            runat="server" Enabled="True" TargetControlID="TextBox1" WatermarkText="ENTER NAME">
        </asp:TextBoxWatermarkExtender>