Establish what information you need from the user in the integration.
Configure custom fields on your product. Make sure they are added to the right sales channel.
Retrieve the API from Backoffice integrations tab.
Use Get Orders API which returns all customer information tied to an order, including custom field values.
Tip: you can for example integrate with loyalty program apps by adding a custom field for the Card ID and managing the integration on your side.
The link will be replaced by new developper portal.
Any new use case using custom fields for external integrations should be detailed here.