I believe you can achieve that with different url, that is dedicated to form processing functionality. Or at least give it a try....
http://localhost:8080/gwt-console-server/rs/form/process/<processId>/complete (POST)
it will try to parse request body to retrieve all paramters and set them as process variables (if they are defined in the process). Unfortunately it will not return process instance id :( as it is dedicated to process forms.
HTH