Can we have an alternative way to pass the custom field while creating a contact.

We have successfully integrated a sync connector between constant contact and our product Vtiger. We are able to sync contacts across products effortlessly with the help of existing API's. But we are facing a problem passing custom fields while creating a contact at the Constant contact end.

As Constant contact expects, custom_field_id to be sent for a particular custom field to be added to a contact record. Our issue is, if the user has multiple custom fields and passing them all for a particular contact record wouldn't be a feasible option as the customer may or may not have those custom field's set to all the contacts. So can you please provide an alternative method to pass on custom fields for contact records.