spring RestTemplate vender specific Media type headers not working

Question

I am trying to convert my jersey api call to spring boot resttemplate client call, when i am trying to add vender specific header its saying unsupported media type. i tried like this

  HttpHeaders headers = new HttpHeaders();
  headers.setAccept(Arrays.asList(MediaType.ALL));
  headers.setContentType(MediaType.ALL); 
  HttpEntity<String> entity = new HttpEntity<String>("parameters", headers);
  ResponseEntity<Admin> result = restTemplate.exchange(url, HttpMethod.GET, entity, Admin.class);

i even tried diffrent combinations of Media types but it did not worked i also tried to add media type like MediaType.parseMediaType("application/vnd.....); please provide some help in this.please let me know what is httpMessage converter and how to add this to our custom vender specific media types.