General help

For future use:

In config.php include the following conde:

function redirect_to_feedback_page_if_available($id, $courseid = SITEID) {
  global $CFG, $PAGE, $USER, $DB;
  require_once($CFG->dirroot . '/mod/feedback/lib.php');
  // Check access to the given courseid.
  if ($courseid AND $courseid != SITEID) {
    return;
  }

  list($course, $cm) = get_course_and_cm_from_cmid($id, 'feedback');
  require_course_login($course, true, $cm);

  $context = context_module::instance($cm->id);
  $feedback = $DB->get_record("feedback", array("id" => $cm->instance), '*', MUST_EXIST);

  $feedbackcompletion = new mod_feedback_completion($feedback, $cm, $courseid);
  if (!has_capability('mod/feedback:edititems', $context) && !$feedbackcompletion->check_course_is_mapped()) {
    return;
  }

  if (!$feedbackcompletion->can_complete()) {
    return;
  }

  // Check if the feedback is open (timeopen, timeclose).
  if (!$feedbackcompletion->is_open()) {
    return;
  }

  // Initialise the form processing feedback completion.
  if ($feedbackcompletion->is_empty() || !$feedbackcompletion->can_submit()) {
    return;
  }

  return "/mod/feedback/complete.php?id=$cm->id";
}

Then edit /login/lib.php, find core_login_get_return_url and add following lines (15313 is the feedback id I want to show users)

    } else if ($feedbackurl = redirect_to_feedback_page_if_available(15313)) {
        $urltogo = $feedbackurl;

after the lines:

    if (user_not_fully_set_up($USER, true)) {
        $urltogo = $CFG->wwwroot.'/user/edit.php';
        // We don't delete $SESSION->wantsurl yet, so we get there later.