Alternative of Basic Auth - Sonar Critical Vulnerability

I need to fetch the access token before making the actual call to the OAuth2-protected API. As of now, I am doing it in the below fashion

String credential = clientId + ":" + clientSecret;
String encoding = Base64.getEncoder().encodeToString((credential).getBytes(‌"UTF‌​-8"​));
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("POST");
conn.setDoOutput(true);
conn.setRequestProperty("Authorization", "Basic " + encoding);
conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
conn.setRequestProperty("charset", "utf-8");
conn.setRequestProperty("Content-Length", Integer.toString(postDataLength));

Sonar says this is Non-Compliant RSPEC-2647, therefore I want to know what are the alternatives I can use, and how to fix this issue?

--EDIT

In the body, I was passing grant_type, username, and password. I tried adding client_id and client_secret as well and it works. Now I have one more question, is passing these credentials in the body more secure?

String body = "grant_type="+grantType+"&username="+username+"&password="+password+"&client_id="+clientId+"&client_secret="+clientSecret;
byte[] postData = body.getBytes(StandardCharsets.UTF_8);
int postDataLength = postData.length;
try(DataOutputStream wr = new DataOutputStream(conn.getOutputStream())) {
    wr.write(postData);
}
conn.connect();