The API for custom fields will be available to your site regardless of whether or not you allow the custom field to be viewed or edited in the Recurly UI.
You can read custom fields on an individual subscription with a GET call.
You can write custom field data to an individual subscription with a PUT or POST call through the API.
The subscription POST call can be made while creating the subscription. This functionality is available beginning in API version 2.14.
<subscription> <custom_fields type="array"> <custom_field> <name>foo</name> <value>asdf</value> </custom_field> </custom_fields> </subscription>
The subscription PUT call functions through the subscriptions/notes route so that it will not collide with other changes being made on the subscription. This functionality is available beginning in API version 2.14
<subscription> <terms_and_conditions>Payment can be sent to Acme Cloud, Inc.</terms_and_conditions> <customer_notes>Thanks for your business!</customer_notes> <vat_reverse_charge_notes>No VAT was applied on this invoice. Please reference this legislation. </vat_reverse_charge_notes> <custom_fields> <custom_field> <name>food</name> <!-- set to the appropriate name --> <value>taco</value> </custom_field> </custom_fields> </subscription>