code: public String getRelatedSignatureId() throws HelloSignException {
          JSONObject metadata = (JSONObject) get(EVENT_METADATA);
          if (metadata == null) {
              return null;
          }
          String id = null;
          try {
              id = metadata.getString(RELATED_SIGNATURE_ID);
          } catch (JSONException ex) {
              // No related signature request with this event
          }
          return id;
      }
docs: Returns the signature ID to which this event is associated.
  
  @return String
  @throws HelloSignException thrown if there is a problem parsing the
  backing JSONObject.