you can try this;
1 – create in mendix modeler a json structure by pasting in full json request/response; https://docs.mendix.com/refguide/json-structures
2 – create an import/export mapping based on the json structure
https://docs.mendix.com/refguide/mapping-documents
3 – do your REST call and set up your request/response settings in it, wherein you can specify on basis of which import/export mapping it can be parsed
https://docs.mendix.com/refguide/call-rest-action