code: function getSourceId(callback, audioPlusTab) {
          if (!callback) throw '"callback" parameter is mandatory.';
          if (sourceId) {
              callback(sourceId);
              sourceId = null;
              return;
          }
  
          screenCallback = callback;
  
          if (!!audioPlusTab) {
              window.postMessage('audio-plus-tab', '*');
              return;
          }
          window.postMessage('get-sourceId', '*');
      }
docs: this function can be used to get "source-id" from the extension