HttpURLConnection header

Question

I am new to user java to connect to server. I have succeed in forming a json which has some key and value to form the body to call the api service and get successful response (with the following code). My questions are:

1. Some of my web service just request header info, for example, I just need to put the ApiKey(key name) and key value in the header and send to server, no other info is needed in body, how can I do this?

2. Some of my web service require both header info and body info, how can I construct the json and send to server? try {

       String input = jsonString;
       URL url = new URL("https://example.com");
       HttpURLConnection conn = (HttpURLConnection) url.openConnection();
       conn.setDoOutput(true);
       conn.setRequestMethod("POST");
       conn.setRequestProperty("Content-Type", "application/json");
       OutputStream os = conn.getOutputStream();
       os.write(input.getBytes());
       os.flush();
       System.out.println("errorcode" + conn.getResponseCode());
   
       if (conn.getResponseCode() != 200) {
           JOptionPane.showMessageDialog(new JFrame(), "Please input a correct username or password");
           return;
       }
   
       BufferedReader br = new BufferedReader(new InputStreamReader(
               (conn.getInputStream())));
   
       String jsonText = read(br);
       System.out.println("jsonText: " + jsonText);