Have you added a Content-Type header to your request?
Use "https://gateway.eu1.mindsphere.io" as domain. You should use the "GatewayUrl" const from the "SiemensInsightsHubSingleSignOn" module.
Provide the authorization header as described here:
https://docs.mendix.com/partners/siemens/mindsphere-development-considerations/#mstoken
and provide a "Content-Type" = "application/json" header.
To figure out the 400 bad request you should check the export mapping, if the exported JSON looks like described in the API documentation:
https://documentation.mindsphere.io/MindSphere/apis/iot-iottimeseries/api-iottimeseries-api.html
I think, you have to adopte your export mapping and remove the "root" element. Simply remove the "checkBox" in the export mapping configuration.