I want to update custom field for subscription via API, for my case field is quantity. I have tried patch to https://api.infusionsoft.com/crm/rest/v1/subscriptions. It seems it not exist. Can anyone help
You will have to use the XML-RPC API, as the REST API has limitations.
The “Data Service” allows you to update the Records.
https://developer.keap.com/docs/xml-rpc/#data-update-a-record
The “RecurringOrder” Table is what you want to use.
https://developer.infusionsoft.com/docs/table-schema/#RecurringOrder
To update the Custom Field, you need to prefix the Name with an Underscore, eg: _ABC
Hope that helps.