JMX is not exposed in a property file. It is toggled from two environment variable. Confluent and Apache Kafka installations share the same variables for this.
Source code here -
Basically, if you export the JMX_PORT to a valid port number, it will open that port for JMX monitoring of any Kafka-related script that you run.
The recommendation, however, is to use a cluster of machines running Connect Distributed, as it is a long-running process, and you can scale your metrics collection using tools like Prometheus JMX Exporter combined with a Grafana server for dashboarding.