The IIH essential expects the data in a dedicated format,the Industrial Edge Common Databus Payload Format. You can find more information about the expected format in the documentation.
To sum up, you require a custom connector with metadata topic, which describes your data and the process data itself. Basically the same topics, which are provided by the official connectors like e.g. the OPC UA Connector .
The following Github example describes the Common Dataload format: https://github.com/industrial-edge/common-databus-payload-format
Additionally you can also check out the following Github example: https://github.com/industrial-edge/opc-ua-application-getting-started
You can find here an example how to create a custom connector using the Flow Creator.
In this example the data will be provided to the OPC UA Server app, but it can be provided in the same way to the IIH Essentials.
https://docs.mendix.com/appstore/connectors/mqtt/ I think you wont find any more than this on the mendix documentation.