Hi Tomohiro,
this should work:
Please note the double opening braces {{ in the request template. This is to ensure that it's not picked up as a parameter.
Hope this helps.
You would usually use a JSON Structure to define you data structure, and then use an Export Mapping to convert the data in your application to that JSON format.
The Mendix Academy learning path on Import and Exporting data has a section on Importing Data Consuming REST Services, which covers exactly what you need.
https://academy.mendix.com/link/paths/44/Importing-and-Exporting-Your-Data
Good luck!