code: @Override
      public String getAuthorizationUrl(OAuthConfig config) {
          Preconditions.checkValidUrl(config.getCallback(),
                  "Must provide a valid url as callback.");
          return String.format(AUTHORIZE_URL, config.getApiKey(),
                  OAuthEncoder.encode(config.getCallback()), SCOPES);
      }
docs: Returns the URL for authorizing the user.
  
  @param config query parameters to add to the base URL.
  
  @return the URL for authorizing the user.