What is your question? What do you want to do?
Given your JSON, you can create a JSON-structure object in your Mendix project. Based on this, you can create an Export Mapping. These 2 only define the structure of the JSON-output and which entities contains the data when exporting.
Before you execute the POST in a REST-activity in your microflow, you need to create data, that is: these 3 objects (or even more) to fill the request. You could also put the Export Mapping as activity in your microflow, that makes debugging the result easier.