All examples of usage Rest API Postman Collection(look at nix-trainee-4.postman_collection.json file).
If you want to send JSON data - you must use Content-Type Header with value application/json. In all casses you will receive data in JSON format except value Header "Access: application/xml" - in this case you will receive XML data
If you want to send XML data - you must use Content-Type Header with value application/xml. If you want to receive XML data - you must use Access Header with value application/xml.