Today, the ioBroker.cloud adapter can be used to properly send POST HTTP requests in JSON format so ioBroker.residents can listen and react to it.

The tendency seems that functionality is being shifted from ioBroker.clouod to the ioBroker.iot adapter, so the ioBroker.residents adapter needs to be prepared for this.

However, the I/O concept of ioBroker.iot is broken as there is no clear separation between GET and POST requests, making it impossible to properly send data. For example, authentication information is always transferred as part of the GET HTTP header payload (like ?token=abc). For complex payload is better transferred using the HTTP body part and POST HTTP request. However, it is not standards compliant to transfer payload in the HTTP header and HTTP body at the same time.
ioBroker.iot currently requires doing this and even though it works in purely handcrafted applications, most frameworks are following the standards and do not allow such crappy mixup.

Unfortunately, there is nothing that can be done here before this gets fixed in the concept ofthe ioBroker.iot adapter. Until then, using the ioBroker.cloud adapter is the only option, even though its functionality might face some deprecation in the near futute.

Also see pending issue here: ioBroker/ioBroker.iot#535