The following solution creates a new user (the response status code should be 201):

Data data = new Data();
data.setId(10);
data.setFirstName("John");
data.setLastName("Year");
data.setAvatar("https://johnyear.com/jy.png");

User newUser = new User();
newUser.setData(data);

HttpRequest requestPost = HttpRequest.newBuilder()
  .header("Content-Type", "application/json")
  .uri(URI.create("https://reqres.in/api/users"))
  .POST(HttpRequest.BodyPublishers.ofString(jsonb.toJson(user)))
  .build();

HttpResponse<Void> responsePost = client.send(
  requestPost, HttpResponse.BodyHandlers.discarding());

int sc = responsePost.statusCode(); // 201

Note that we ignore any response body via HttpResponse.BodyHandlers.discarding().