OkHttpClient client = new OkHttpClient();

// A section here gets strings from a JSON file storing values such as client_id

 RequestBody bodyBuilder = new FormBody.Builder()
  .add("client_secret", CLIENT_SECRET)
  .add("client_id", CLIENT_ID)
  .add("code", AUTHORIZATION_CODE)
  .add("grant_type", GRANT_TYPE)
  .add("redirect_uri", REDIRECT_URI)
  .build();
 System.out.println("Built body: " + bodyBuilder.toString());

 String mediaTypeString = "application/x-www-form-urlencoded";
 MediaType mediaType = MediaType.parse(mediaTypeString);
 RequestBody body = RequestBody.create(mediaType, requestbodyToString(bodyBuilder)); // See Edit 1

 Request request = new Request.Builder()
  .url(TARGET_URL)
  .post(body)
  .addHeader("content-type", mediaTypeString)
  .addHeader("cache-control", "no-cache")
  .build();

 try {
  System.out.println("Starting request.");
  Response response = client.newCall(request).execute();
  String targetUrl = request.url().toString() + bodyToString(request);
  System.out.println("request: " + targetUrl);
  String responseBodyString = response.body().string();
  System.out.println("response: " + responseBodyString);
  return responseBodyString;
 } catch (IOException ex) {
  System.out.println(ex);
 }