Getting 403 forbidden for access token in java

String credentials = clientId + ":" + clientSecret;
String authHeader = "Basic " + Base64.getEncoder().encodeToString(credentials.getBytes());
System.out.println("authHeader==>" + authHeader);
// Create request URL
StringBuilder requestBuilder = new StringBuilder()
.append("https://authz.constantcontact.com/oauth2/default/v1/token").append("?code=")
.append(authCode).append("&redirect_uri=").append(redirectUri)
.append("&grant_type=authorization_code");
System.out.println("url==>" + requestBuilder.toString());

MultiValueMap<String, String> requestBody = new LinkedMultiValueMap<>();
HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
headers.set("Accept","application/json");
headers.set("Authorization", authHeader);
HttpEntity<MultiValueMap<String, String>> entity = new HttpEntity<>(requestBody, headers);
ResponseEntity<String> response = restTemplate.exchange(requestBuilder.toString(), HttpMethod.POST, entity, String.class);
System.out.println("response==>"+response);