webapp/templates/default/module/room.html.twig

  <form name="room" action="{{ path('room_add', { mode : request.get('mode') }) }}" method="post">
      {% if request.get('error') %}
          <output name="error" for="form-room-name">{{ request.get('error') }}</output>
      {% endif %}
      <input type="text" name="name" id="form-room-name" value="{{ request.get('name') }}" placeholder="{{ 'enter new room name...' | trans }}" />
      <button type="submit">{{ 'add' | trans }}</button>
  </form>