code: @SuppressWarnings("WeakerAccess")
      public Request<List<UserIdentity>, ManagementException> link(String primaryUserId, String secondaryToken) {
          HttpUrl url = HttpUrl.parse(auth0.getDomainUrl()).newBuilder()
                  .addPathSegment(API_PATH)
                  .addPathSegment(V2_PATH)
                  .addPathSegment(USERS_PATH)
                  .addPathSegment(primaryUserId)
                  .addPathSegment(IDENTITIES_PATH)
                  .build();
  
          final Map<String, Object> parameters = ParameterBuilder.newBuilder()
                  .set(LINK_WITH_KEY, secondaryToken)
                  .asDictionary();
  
          TypeToken<List<UserIdentity>> typeToken = new TypeToken<List<UserIdentity>>() {
          };
          return factory.POST(url, client, gson, typeToken, mgmtErrorBuilder)
                  .addParameters(parameters);
      }
docs: Link a user identity calling <a href="https://auth0.com/docs/link-accounts#the-management-api">'/api/v2/users/:primaryUserId/identities'</a> endpoint
  Example usage:
  <pre>
  {@code
  client.link("{auth0 primary user id}", "{user secondary token}")
  .start(new BaseCallback<List<UserIdentity>, ManagementException>() {
  {@literal}Override
  public void onSuccess(List<UserIdentity> payload) {}
  
  {@literal}Override
  public void onFailure(ManagementException error) {}
  });
  }
  </pre>
  
  @param primaryUserId  of the identity to link
  @param secondaryToken of the secondary identity obtained after login
  @return a request to start