<div class="form-group">
  <label for="kursort">Kursort</label>
  <select id="kursort" name="kursort_id" class="form-control single-select" required>
    <option value="">Bitte wählen</option>
    {% for item in params.veranstalter %}
      <optgroup label="{{ item.veranstalter }}" data-id="{{ item.id }}">
        {% for kursort in item.kursorte %}
          {% set temp = kursort|split('|') %}
          {% if temp.1 is defined %}
            {% set selected = '' %}
            {% if element.kursort_id == temp.0 %}
              {% set selected = ' selected' %}
            {% endif %}
            <option value="{{temp[0]}}"{{ selected }}>{{temp[1]}}</option>
          {% endif %}
        {% endfor %}
      </optgroup>
    {% endfor %}
  </select>
</div>