code: private BackchannelAuthenticationResponse callBackchannelAuthentication(
              String parameters, String clientId, String clientSecret,
              String clientCertificate, String[] clientCertificatePath)
      {
          if (parameters == null)
          {
              // Authlete returns different error codes for null and an empty string.
              // 'null' is regarded as a caller's error. An empty string is regarded
              // as a client application's error.
              parameters = "";
          }
  
          // Create a request for Authlete's /api/backchannel/authentication API.
          BackchannelAuthenticationRequest request = new BackchannelAuthenticationRequest()
              .setParameters(parameters)
              .setClientId(clientId)
              .setClientSecret(clientSecret)
              .setClientCertificate(clientCertificate)
              .setClientCertificatePath(clientCertificatePath)
              ;
  
          try
          {
              // Call Authlete's /api/backchannel/authentication API.
              return mApi.backchannelAuthentication(request);
          }
          catch (AuthleteApiException e)
          {
              // The API call failed.
              throw apiFailure("/api/backchannel/authentication", e);
          }
      }
docs: Call Authlete's {@code /api/backchannel/authentication} API.