Creating a body from InputStream can be accomplished using BodyPublishers.ofInputStream(), as shown in the following snippet of code (here, we rely on ByteArrayInputStream but, of course, any other InputStream is suitable):

HttpRequest requestBodyOfInputStream = HttpRequest.newBuilder()
  .header("Content-Type", "application/json")
  .POST(HttpRequest.BodyPublishers.ofInputStream(()
    -> inputStream("user.json")))
  .uri(URI.create("https://reqres.in/api/users"))
  .build();

private static ByteArrayInputStream inputStream(String fileName) {

  try (ByteArrayInputStream inputStream = new ByteArrayInputStream(
    Files.readAllBytes(Path.of(fileName)))) {

    return inputStream;
  } catch (IOException ex) {
    throw new RuntimeException("File could not be read", ex);
  }
}

In order to take advantage of lazy creation, InputStream has to be passed as Supplier.