code: function getCustomSourceId(arr, callback) {
      if (!arr || !arr.forEach) throw '"arr" parameter is mandatory and it must be an array.';
      if (!callback) throw '"callback" parameter is mandatory.';
  
      if (sourceId) return callback(sourceId);
  
      screenCallback = callback;
      window.postMessage({
          'get-custom-sourceId': arr
      }, '*');
  }
docs: this function can be used to get "source-id" from the extension