Use the findOrCreate.linkToEntity method to link the record you found or created to another record type.
Usage
Links the record that you found or created using the findOrCreate and findOrCreate.map Deployment API methods to another record of a different record type that you created using a separate findOrCreate API call. For example, you can link a case record you found within your organization to a contact record you create.
You can only link records if the parent record is created with a findOrCreate API call. You can’t link a child record to a record you found using the findOrCreate.linkToEntity method.