Trying to get an Access Token from Dwolla restful api - java

Im having trouble getting an access token for the sandbox environment.
Im following this guide for authenticating: OAuth
So when i create my request, following this guide, i get the following response from the api:
{"error":"access_denied","error_description":"Invalid application credentials."}
Im using key for mf client ID and secret as my client secret as per the instructions.
Here is the code Im using:
public static void main(String[] args) {
try {
URL url = new URL("https://www.dwolla.com/oauth/v2/token");
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
conn.setRequestProperty("client_id", "<Key>");
conn.setRequestProperty("client_secret", "<Secret>");
conn.setRequestProperty("grant_type", "client_credentials");
conn.setDoInput(true);
conn.setDoOutput(true);
System.out.println("Message:" + conn.getResponseMessage());
BufferedReader br = new BufferedReader(new InputStreamReader(
(conn.getInputStream())));
String output;
System.out.println("Output from Server .... \n");
while ((output = br.readLine()) != null) {
System.out.println(output);
}
conn.disconnect();
} catch (MalformedURLException ex) {
Logger.getLogger(PaymentTest.class.getName()).log(Level.SEVERE, null, ex);
} catch (IOException ex) {
Logger.getLogger(PaymentTest.class.getName()).log(Level.SEVERE, null, ex);
}
}

I was able to get an access token finally. My problem is first of all the above code uses client_id and client_secret as header params. These need to go in the body of the request.
My second problem is that I used the wrong content type for the message I was sending.
Here is the code that worked for me:
URL url = new URL("https://sandbox.dwolla.com/oauth/v2/token");
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type", "application/json");
conn.setDoInput(true);
conn.setDoOutput(true);
String data = "";
JSONObject jsonObj = new JSONObject();
jsonObj.put("client_id", "<Your Client ID>");
jsonObj.put("client_secret", "<Your Client Secret>");
jsonObj.put("grant_type", "client_credentials");
data = jsonObj.toString();
System.out.println("data = " + data);
byte[] outputInBytes = data.getBytes("UTF-8");
OutputStream os = conn.getOutputStream();
os.write( outputInBytes );
os.close();
System.out.println("Message:" + conn.getResponseMessage());
BufferedReader br = new BufferedReader(new InputStreamReader((conn.getInputStream())));
String output;
System.out.println("Output from Server .... \n");
while ((output = br.readLine()) != null) {
System.out.println(output);
}
conn.disconnect();

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With this second code I'm not getting that error, but there is no inserting to the database(checking that using postman, with GET method)...
Well, what am I missing? I guess, I'm missing something basic...
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