This is a regression in 7.5.
It is no longer possible to call the POST /message endpoint without supplying a business key for correlation.
The reason is that the message rest service always calls the #businessKey parameter on the message correlation builder (see https://github.com/camunda/camunda-bpm-platform/blob/7.5.0/engine-rest/engine-rest/src/main/java/org/camunda/bpm/engine/rest/impl/MessageRestServiceImpl.java#L71-L74) and the correlation builder now has a not null check (see https://github.com/camunda/camunda-bpm-platform/blob/7.5.0/engine/src/main/java/org/camunda/bpm/engine/impl/MessageCorrelationBuilderImpl.java#L72).
- use Java API
- write custom REST API