DECLARE

  l_result          clob;
  l_client_id       VARCHAR2(300) := 'client_id_hidden';
  l_client_secret   VARCHAR2(300) := 'client_secre_hidden';

  FUNCTION base64encode(t IN VARCHAR2) RETURN VARCHAR2 IS
  BEGIN
    RETURN TRANSLATE(UTL_RAW.cast_to_varchar2(UTL_ENCODE.base64_encode(UTL_RAW.cast_to_raw(t))), 'd'||CHR(10)||CHR(13), 'd');
  END;

BEGIN

  apex_web_service.g_request_headers.DELETE();

  apex_web_service.g_request_headers(1).name  := 'authorization';
  apex_web_service.g_request_headers(1).VALUE := 'Basic '|| base64encode(l_client_id||':'||l_client_secret);
  apex_web_service.g_request_headers(2).name  := 'Content-Type';
  apex_web_service.g_request_headers(2).VALUE := 'application/x-www-form-urlencoded';

  l_result := apex_web_service.make_rest_request(
    p_url            => 'http://localhost/identity-xero-api-proxy/connect/token'
   ,p_http_method    => 'POST'
   ,p_body           => '{"grant_type":"client_credentials"
                         ,"scope":"accounting.transactions accounting.contacts assets files"
                         }'
   );

  DBMS_OUTPUT.put_line(l_result);

END;