Using RequiredfieldValidator on a TextBox having JavaScript WaterMark in ASP.Net

hi

I use below code for textbox watermark:

<script type="text/javascript">
    var defaultText = "Enter Your Text here";
    function WaterMark(txt, evt) {
        if (txt.value.length == 0 && evt.type == "blur") {
            txt.style.color = "#b9b9b9";
            txt.value = defaultText;
        }
        if (txt.value == defaultText && evt.type == "focus") {
            txt.style.color = "black";
            txt.value = "";
        }
    }
</script>

and

  <asp:TextBox ID="Txtgaranti" runat="server" CssClass="txtgaranti1" Text="Enter Your Text here"
                                        ForeColor="#b9b9b9" onblur="WaterMark(this, event);" onfocus="WaterMark(this, event);"></asp:TextBox>

now I want use RequiredFieldValidator for this textbox but it doesn't work I think it is becuse of this text "Enter Your Text Here" so what should I do?

Best Regards

Neda