code: private BackchannelAuthenticationFailResponse callBackchannelAuthenticationFail(String ticket, BackchannelAuthenticationFailRequest.Reason reason)
      {
          // Create a request for /api/backchannel/authentication/fail API.
          BackchannelAuthenticationFailRequest request = new BackchannelAuthenticationFailRequest()
              .setTicket(ticket)
              .setReason(reason)
              ;
  
          try
          {
              // Call Authlete's /api/backchannel/authentication/fail API.
              return mApi.backchannelAuthenticationFail(request);
          }
          catch (AuthleteApiException e)
          {
              // The API call failed.
              throw apiFailure("/api/backchannel/authentication/fail", e);
          }
      }
docs: Call Authlete's {@code /api/backchannel/authentication/fail} API.