Custom fields right now are available only for the Customer object. If you would like to retrieve the custom field data, you can use the Retrieve a Customer API. The API response will contain the custom fields, if it is present for that Customer.

Sample Code:

Create Subscription API:

EntityResult result = Subscription.Create ()
 .PlanId ("basic")
 .CustomerEmail ("john@user.com")
 .CustomerFirstName ("John")
 .CustomerLastName ("Doe")
 .Param("customer[cf_client_id]", "xyz")
 .Request();

The custom field values can be sent using param method with "customer[cf_<attribute_name>]".

Retrieve Custom field value if present in API response:

Customer customer = Customer.Retrieve("<customer_id>").Request().Customer;
Console.WriteLine (customer.GetValue<String> ("cf_client_id", false));

The custom field value can be accessed using GetValue<String> ("cf_client_id", false) method. Here, false parameter specifies not to throw error if the API response does not have the custom field value. By default, if the boolean parameter is not sent it will be considered as 'true'.

For other languages, please refer here: https://www.chargebee.com/tutorials/custom-fields-recurring-billing-example.html.