The addon cannot be configured from the addon configuration page. In order to configure the addon, you need to add/edit the /config/risco-mqtt.json file and restart the addon.
To create and edit the configuration file, you can use Studio Code Server addon or Home assistant File Editor.
The addon just extends the risco-mqtt-local docker image. Therefore, if you make changes to the source code of the risco-mqtt (and not the addon itself), you will need to rebuild the risco-mqtt image.