netbox/netbox/utilities/templates/widgets/select_api.html

<select name="{{ widget.name }}"{% include "django/forms/widgets/attrs.html" %}>{% for group_name, group_choices, group_index in widget.optgroups %}{% if group_name %}
  <optgroup label="{{ group_name }}">{% endif %}{% for widget in group_choices %}{% if widget.attrs.selected %}
  {% include widget.template_name %}{% endif %}{% endfor %}{% if group_name %}
  </optgroup>{% endif %}{% endfor %}
</select>