//GET THE PRODUCT ID FROM THE URL
            Product product = _productRepo.Load(productId);

            ProductCustomField pcf = new ProductCustomField();
            pcf.Product = product;
            pcf.IsUserDefined = true;
            pcf.IsVisible = false;
            pcf.FieldName = "Internal Tracking Number";
            pcf.FieldValue = "XYZ 123";
            product.CustomFields.Add(pcf);
            _productRepo.Save(product);

            var customField = product.CustomFields
                .Where(field => field.FieldName == "Internal Tracking Number")
                .FirstOrDefault();
            string internalTrackingNumber = customField.FieldValue;