code: public Request<List<Identity>> linkIdentity(String primaryUserId, String secondaryUserId, String provider, String connectionId) {
          Asserts.assertNotNull(primaryUserId, "primary user id");
          Asserts.assertNotNull(secondaryUserId, "secondary user id");
          Asserts.assertNotNull(provider, "provider");
  
          String url = baseUrl
                  .newBuilder()
                  .addPathSegments("api/v2/users")
                  .addPathSegment(primaryUserId)
                  .addPathSegment("identities")
                  .build()
                  .toString();
  
          CustomRequest<List<Identity>> request = new CustomRequest<>(client, url, "POST", new TypeReference<List<Identity>>() {
          });
          request.addHeader("Authorization", "Bearer " + apiToken);
          request.addParameter("provider", provider);
          request.addParameter("user_id", secondaryUserId);
          if (connectionId != null) {
              request.addParameter("connection_id", connectionId);
          }
          return request;
      }
docs: Links two User's Identities.
  A token with scope update:users is needed.
  See https://auth0.com/docs/api/management/v2#!/Users/post_identities
  
  @param primaryUserId   the primary identity's user id
  @param secondaryUserId the secondary identity's user id
  @param provider        the provider name of the secondary identity.
  @param connectionId    the connection id of the secondary account being linked, useful if the provider is 'auth0' and you have several connections. Can be null.
  @return a Request to execute.