IBM Connections Cloud custom contextual data

Question

I followed the official documentation and created a minor community app for my website.

What I'd like to know is if there's a way "to shim" custom data to IBM Connections Cloud when uploading my app as a JSON and then retrieve it inside the message handler after my app loads:

window.addEventListener('message', function(event) {

  if (event.origin === "https://apps.collabservnext.com"){
    // event.data contains the full context
    console.log("Running in community named " + event.data.source.resourceId);
  }
}, false);

parent.postMessage("appReady", "*");

So far I'm only getting the extraContent, source and user JSON keys in event.data. It would be awesome if there was a way to retrieve say hello: world.

Thanks.