To set custom headers ON A REQUEST, build a request with the custom header before passing it to httpclient to send to http server. eg:
HttpClient client = HttpClients.custom().build();
HttpUriRequest request = RequestBuilder.get()
.setUri(someURL)
.setHeader(HttpHeaders.CONTENT_TYPE, "application/json")
.build();
client.execute(request);
Default header is SET ON HTTPCLIENT to send on every request to the server.