<link href="/path/to/bootstrap.css" rel="stylesheet" />
<script src="/path/to/jquery.js"></script>
<script src="/path/to/bootstrap.js"></script>
<script src="/path/to/regular.js"></script>
<script src="/path/to/fontawesome.js"></script>
<script src="/path/to/multiselect-bs4.js"></script>

<div class="row justify-content-center">
<div class="form-group col-6">
<label for="color">Color:</label>
<select id="color" class="multiselect" name="color" multiple="multiple">
<option value="1">Red</option>
<option value="2">Orange</option>
<option value="3">Yellow</option>
<option value="4">Green</option>
<option value="5">Blue</option>
<option value="6">Indigo</option>
<option value="7">Violet</option>
<optgroup label="Reds">
<option value="8">Light Red</option>
<option value="9">Dark Red</option>
</optgroup>
</select>
</div>
</div>

jQuery('.multiselect').multiselect();