themes/flamecore/default/templates/ui/form_field_multiselect.twig
<div class="form-group{% if submitted and field.name in invalid %} has-error{% endif %}">
<label for="{{ field.name }}">{{ field.title }}{% if field.required %} <em class="required">*</em>{% endif %}</label>
<select name="{{ field.name }}[]" id="{{ field.name }}" multiple{% if field.size %} size="{{ field.size }}"{% endif %} class="{{ field.class|default('form-control') }}">
{% for value, caption in field.options %}
<option value="{{ value }}"{% if value in (submitted ? data[field.name] : field.value) %} selected="selected"{% endif %}>{{ caption }}</option>
{% endfor %}
</select>
{% if field.errorText and submitted and field.name in invalid %}<p class="help-block error-text">{{ field.errorText }}</p>{% endif %}
{% if field.description %}<p class="help-block">{{ field.description }}</p>{% endif %}
</div>