netbox-community-netbox/netbox/templates/django/forms/widgets/select.html

<select name="{{ widget.name }}"{% include "django/forms/widgets/attrs.html" %} class="{% if 'size' in widget.attrs %}form-select form-select-sm{% else %}netbox-static-select{% endif %}{% if 'class' in widget.attrs %} {{ widget.attrs.class }}{% endif %}">{% for group_name, group_choices, group_index in widget.optgroups %}{% if group_name %}
  <optgroup label="{{ group_name }}">{% endif %}{% for option in group_choices %}
  {% include option.template_name with widget=option %}{% endfor %}{% if group_name %}
  </optgroup>{% endif %}{% endfor %}
</select>