code: public String getAppId() throws HelloSignException {
          JSONObject metadata = (JSONObject) get(EVENT_METADATA);
          String id = null;
          try {
              id = metadata.getString(REPORTED_FOR_APP_ID);
          } catch (JSONException ex) {
              throw new HelloSignException(ex);
          }
          return id;
      }
docs: Returns the API app ID for which this event is reported.
  
  @return String
  @throws HelloSignException thrown if there is a problem parsing the
  backing JSONObject.