Symfony Form Builder Bootstrap 4 input-group Append and Prepend

my_bootstrap_4_layout.html.twig

{% use "bootstrap_4_layout.html.twig" %}

{# extend form_widget_simple #}
{% block form_widget_simple -%}
    {%- if (prepend is defined) or (append is defined) -%}        
        <div class="input-group{{ group_class|default('') }}">
            {%- if prepend is defined -%}
                <div class="input-group-prepend">
                    {{ prepend|raw }} {# without raw filter the html code will be escaped #}
                </div>
            {%- endif -%}
            {{- parent() -}}
            {%- if append is defined -%}
                <div class="input-group-append">
                    {{ append|raw }}
                </div>
            {%- endif -%}
        </div>
    {%- else -%}
        {{- parent() -}}
    {%- endif -%}
{%- endblock form_widget_simple %}

usage.html.twig

{% form_theme form 'my_bootstrap_4_layout.html.twig' %}
{{ form_start(form) }}
{{ form_row(form.facebook, {'prepend': '<button class="btn btn-icon btn-facebook"><i class="fab fa-facebook-f"></i></button>'}) }}
{{ form_row(form.area, {'append': '<span class="input-group-text" id="basic-addon1">m²</span>'}) }}
{{ form_end(form) }}