Read custom neo4j.conf settings from extension

We authored a custom Transaction Listener Extension following this example. It's been packed and deployed as a plugin to a local Neo4j 4.2.1 instance.

We also added a custom setting to the corresponding neo4j.conf configuration as key-value-pair, but somehow this settings won't be picked up by Neo4j. The Cypher command dbms.listConfig()doesn't return the custom setting. Inside the Extension, the call to org.neo4j.configuration.Config.getDeclaredSettings() doesn't return it either.

Is there any other configuration necessary that Neo4j picks up custom settings, or maybe a specific naming convention?