Hi,
please check in HELP -> B1 DI-API Service for the method templates of the MessagesService service call.
This documentation might not exist in older B1i versions, here are the screenshots of an B1 9.1 version:
sendMessage:
From my point of view it looks like you defined the service method "sendMessage" in the maintenance UI, but in the payload you used the service method "getMessage".
This might explain the odd DI API error message. It would be interesting to see a screenshot of your atom19's payload.
I attached an example of a sendMessage definition with an according payload:
Best regards
Bastian