the rule.setUniqueKey(String repositoryKey, String key) also feed the mandatory configKey property with the value of the key

sonar-plugin-api/src/main/java/org/sonar/api/rules/Rule.java

   }
 
   public Rule setUniqueKey(String repositoryKey, String key) {
-    return setRepositoryKey(repositoryKey).setKey(key);
+    return setRepositoryKey(repositoryKey).setKey(key).setConfigKey(key);
   }
 
   public Cardinality getCardinality() {