Get related fields from Salesforce objects using Amazon Appflow

Hi, I'm trying to create a flow between Salesforce and Amazon S3 in order to get ready-to-use reports. For that purpose, I need to get related fields of the objects I'm importing. For example, I have an Invoice object which relates to a Contract (one contract may have multiple invoices), and in my report, I would need to get an external id of the contract.

Using SOQL and the Bulk API, I can retrieve this field using the following query: SELECT ID_Contrato__r.External_ID__c FROM Invoice__c, therefore it is possible to get related fields using this API (which can be selected to be used by Appflow). However, in the Appflow console, I can't see any option that allows me to get those related fields. Could you please tell me how can I retrieve them? And if it is not possible, could you explain me why, as the Salesforce API does allow it?

Thank you