The easiest approach will be to create a JSON Structure for the expected output that API expects, and use an Export Mapping to convert your data to JSON. You can use the Export Mapping in the Call REST action you use to send the data.
https://docs.mendix.com/refguide/json-structures/
https://docs.mendix.com/refguide/export-mappings/
https://docs.mendix.com/refguide/call-rest-action/
I hope this helps.