You need to have an xsd file or web service defined to start the mapping.
One way to create an xsd is to first create a microflow that returns a single object of the type you want to export, then publish the microflow as a webservice. In the webservice operation return type, you can select which elements to include. Once this is done you can go to the settings tab of the published web service where you can export it as an XSD.
You can then manually edit the xsd if necessary, and use it in defining your export mapping.
From Documentation for v6:
Importing from JSON is available since Mendix 6.6.0. Exporting to JSON will be made available in a later Mendix version.