Section "5.3. Creating service tasks" describes how to use the BPMN ServiceTask but it does not mention the required parameters a user must pass to get a Java ServiceTask correctly invoked. Please, see attached screenshot showing an example of how to pass these parameters in order to invoke a Java ServiceTask. 

Also see these params in the src code: https://github.com/kiegroup/jbpm/blob/main/jbpm-workitems/jbpm-workitems-bpmn2/src/main/java/org/jbpm/process/workitem/bpmn2/ServiceTaskHandler.java#L282-L283

https://access.redhat.com/documentation/en-us/red_hat_process_automation_manager/7.11/html-single/developing_process_services_in_red_hat_process_automation_manager/index#create-service-task-proc