Please don’t.
- it requires and only works with the MQTT 1.x binding which is end of life and will not be supported by OH 3
- it’s write only and not queryable
- it is full of bugs and essentially was abandonded when the MQTT Event Bus was developed for MQTT 1.x something like seven years ago.
If you want to use MQTT to publish Item updates over MQTT, a better approach is to use the 2.x binding: Marketplace MQTT Event Bus.