{% load widget_tweaks %}

<form id="user-select" action="" method="get">
  {% for hidden_field in form.hidden_fields %}
    {{ hidden_field }}
  {% endfor %}
  <div class="form-group">
    <div class="d-flex">
      {{ form.user|add_class:"form-select" }}
      <noscript>
        <button type="submit" class="btn btn-link ml-2">Apply</button>
      </noscript>
    </div>
  </div>
</form>
<script>
  const form = document.getElementById('user-select');
  const select = form.querySelector('select');
  select.addEventListener('change', () => {
    form.submit();
  });
</script>