<form>

  <select class="select" multiple required>

    <option value="1">One</option>

    <option value="2">Two</option>

    <option value="3">Three</option>

    <option value="4">Four</option>

    <option value="5">Five</option>

    <option value="6">Six</option>

    <option value="7">Seven</option>

    <option value="8">Eight</option>

  </select>

  <button type="submit" class="btn btn-primary mt-3">

    Submit

  </button>

</form>

​