Custom Validation for checking empty textbox

Hello,
I am using ASP.NET c# Custom validator control to check whether the user has entered the "name shown on the credit card" information in the textbox or not, ONLY IF he chose Credit card payment as the payment method.
On the ASP.NET webform, I have a RadioButtonList control with ID "rdPaymentMethod" for payment method selection and also a textbox with ID "txtCCName" for entering the name shown on the credit card.
<p> <asp:RadioButtonList ID="rdPaymentMethod" runat="server"> <asp:ListItem Value="Credit Card"> Credit Card</asp:ListItem> <asp:ListItem Value="Cheque"> PayPal</asp:ListItem> <asp:ListItem Value="Cash"> Cheque</asp:ListItem> </asp:RadioButtonList> </p> <p> ...