code: public OAuth1RequestToken getRequestToken(String callbackUrl) {
          String callback = (callbackUrl != null) ? callbackUrl : OUT_OF_BOUND_AUTH_METHOD;
  
          OAuth10aService service = new ServiceBuilder(apiKey)
                  .apiSecret(sharedSecret)
                  .callback(callback)
                  .build(FlickrApi.instance());
  
          try {
              return service.getRequestToken();
          } catch (IOException | InterruptedException | ExecutionException e) {
              throw new FlickrRuntimeException(e);
          }
      }
docs: Get the OAuth request token - this is step one of authorization.
  
  @param callbackUrl
  optional callback URL - required for web auth flow, will be set to "oob" if not specified.
  @return the {@link OAuth1RequestToken}, store this for when the user returns from the Flickr website.