link in label.twig

{% form_theme register  _self %}

{% block checkbox_row %}
    {% spaceless %}
        <div class="form-group">
            <div class="checkbox">
                {{ form_errors(form) }}
                <label class="control-label required" for="{{ form.vars.id }}">
                    <input type="checkbox" {{ block('widget_attributes') }} {% if value is not empty %}value="{{ value }}" {% endif %}/>
                    {{ label|default(form_label(form)) | raw }}
                </label>
            </div>
        </div>
    {% endspaceless %}
{% endblock %}


{% block checkbox_widget -%}
    {% set parent_label_class = parent_label_class|default('') -%}
    {% if 'checkbox-inline' in parent_label_class %}
        {{- form_label(form, null, { widget: parent() }) -}}
    {% else -%}
        <div class="checkbox">
            {{- form_label(form, null, { widget: parent() }) -}}
        </div>
    {%- endif %}
{%- endblock checkbox_widget %}