display cutom field for a customer

Hi, I am trying to display information about our customer based on internal ID in C#. It work fine as far as customer name, email, phone.... but I ran into some issue displaying the custom field of specific customer
here is what I have so far:
RecordRef customer = new RecordRef();
                    customer.internalId = "517";
                    customer.type = RecordType.customer;
                    customer.typeSpecified = true;
                    var result = _service.get(customer);
                    Customer cus = result.record as Customer;
                   output.Text = cus.companyName + "   -           " + cus.category.name

please let me know how I can display a custom field for this customer.
Thanks for any help or advice.
Dean