Supported intent parameters for INPlayMediaIntent

I created an iOS App Extension and defined a single entry INPlayMediaIntent in the extension's plist IntentsSupported array. Everything was fine. But since a few days (WWDC 2019) i have trouble submitting the app to TestFlight/App Store Connect.

I followed Apple's instructions and succuessfully fixed these errors:

• ITMS-90626: Invalid Siri Support - No example phrase was provided for INPlayMediaIntent in the … language. Please refer to "https://developer.apple.com/documentation/sirikit/registering_custom_vocabulary_with_sirikit/global_vocabulary_reference/intent_phrases
• ITMS-90626: Invalid Siri Support - No intent parameters specified for bundle … language … vocabulary policy #1

Now I'm trying to provide an intent parameter in the AppIntentVocabulary.plist here:

  ParameterVocabularies
  
  
  ParameterNames
  
  INPlayMediaIntent.mediaItems
   

I tried it with various strings in ParameterNames and all were wrong according to these App Store emails:

• ITMS-90626: Invalid Siri Support - "INPlayMediaIntent.mediaItems" is not a supported intent parameter of the subscribed intents
• ITMS-90626: Invalid Siri Support - "INPlayMediaIntent.mediaContainer" is not a supported intent parameter of the subscribed intents
• ITMS-90626: Invalid Siri Support - "INPlayMediaIntent.identifier" is not a supported intent parameter of the subscribed intents

What would be a valid parameter name for an INPlayMediaIntent here?

It could be so easy because Apple has an example project for INPlayMediaIntent here which i used to learn for my project:

https://developer.apple.com/documentation/sirikit/media/playing_media_through_siri_shortcuts?language=objc

But: This project seems not to be up to date since it is missing the AppIntentVocabulary.plist which seems to be required recently. Maybe Apple can update that project example to make things clear again?