// without configPrefix
return new kibana.Plugin({
  id: 'myPlugin',
  config: function (Joi) {
    return Joi.object({
      mySetting: Joi.boolean().default(true),
    });
  },
  init: function (server, options) {
    const config = server.config();
    config.get('myPlugin.mySetting'); // returns true
  }
});

// with custom configPrefix
return new kibana.Plugin({
  id: 'myPlugin',
  configPrefix: 'customPluginPrefix',
  config: function (Joi) {
    return Joi.object({
      mySetting: Joi.boolean().default(true),
    });
  },
  init: function (server, options) {
    const config = server.config();
    config.get('customPluginPrefix.mySetting'); // returns true
  }
});