The way I understand it is when you send a request to the REST API, you're going to get back a response which is predefined such as the process instance ID or maybe even just an HTTP status code. You can't really control that very easily. So, whatever you do will probably have to exist outside of the standard BPMN tools. I'm sure someone out there has probably implemented this.
You should check into the examples on Github at this URL: https://github.com/camunda/camunda-bpm-examples
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