Hi..
See this,its may useful to u.
<asp:TextBox ID="TextBox6" runat="server"></asp:TextBox>
<asp:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender4" runat="server"
TargetControlID="TextBox6" WatermarkCssClass="Water Mark"
WatermarkText="Amount In Rs....">
</asp:TextBoxWatermarkExtender>
<asp:RequiredFieldValidator ID="reqfieldvaldatorDepositForReg" runat="server"
ControlToValidate="TextBox6" Display="Dynamic"
ErrorMessage="Deposit should not be empty" ForeColor="#CC0000"
ValidationGroup="pnl"></asp:RequiredFieldValidator>
Use like this,Thank u.