How to send a proper post request to WHMCS installation from android app?

I am trying to send an AddClient post request to my WHMCS installation through my android app but it keeps returning "result=error;message=You did not enter your first name".

I get it to authenticate successfully but it doesnt seem to be posting the jsonObject I am sending it.

Here is a link to WHMCS api documentation:

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public void createWHMCSUser(final String emailID, final String random, final String uid) {

final String AccessUserKey = "abc123";
final String AccessKey = "abc123";
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
try {
URL url = new URL("http://whmcs.installation.com/includes/api.php?action=AddClient&username=abc123&password=abc123&accesskey=abc123");
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type", "application/json");
conn.setRequestProperty("Accept","application/json");
conn.setDoOutput(true);
conn.setDoInput(true);

JSONObject jsonParam = new JSONObject();
jsonParam.put("action", "AddClient");
jsonParam.put("identifier", AccessUserKey);
jsonParam.put("secret", AccessKey);
jsonParam.put("firstname", "User");
jsonParam.put("lastname", "Name");
jsonParam.put("email", emailID);
jsonParam.put("address1", "na");
jsonParam.put("city", "na");
jsonParam.put("state", "na");
jsonParam.put("postcode", "00000");
jsonParam.put("country", "US");
jsonParam.put("phonenumber", "0000000000");
jsonParam.put("password2", random);
jsonParam.put("repsonsetype", "json");

Log.i("JSON", jsonParam.toString());
DataOutputStream os = new DataOutputStream(conn.getOutputStream());
os.writeBytes(URLEncoder.encode(jsonParam.toString(), "UTF-8"));
//os.writeBytes(jsonParam.toString());

os.flush();
os.close();

Log.i("STATUS", String.valueOf(conn.getResponseCode()));
Log.i("MSG", conn.getResponseMessage());



conn.disconnect();
} catch (Exception e) {
e.printStackTrace();
}

}
});

thread.start();

}

It should return "result=success".

But actually returns "result=error;message=You did not enter your first name".