Example

$form = new Form('plugins-dependent-fields-form-1', 'horizontal');

$form->addOption('subject', '', 'Your request concerns:');
$form->addOption('subject', 'Support', 'Support');
$form->addOption('subject', 'Other', 'Other');
$form->addHelper('if other, please tell us more', 'subject');
$form->addSelect('subject');

$form->startDependentFields('subject', 'Other');
$form->addInput('text', 'request-more', '', '', 'required, placeholder=Please tell more about your request ...');
$form->endDependentFields();

<div class="row">
    <div class="offset-sm-4 col-sm-8 mb-3">
        <select id="subject" name="subject" class="form-select">
            <option value="" >Your request concerns:</option>
            <option value="Support" >Support</option>
            <option value="Other" >Other</option>
        </select>
        <span class="form-text" id="subject-helper">if other, please tell us more</span>
    </div>
</div>
<div class="hidden-wrapper off" data-parent="subject" data-show-values="Other" data-inverse="">
    <div class="row">
        <div class="offset-sm-4 col-sm-8 mb-3">
            <input id="request-more" name="request-more" type="text" value="" required placeholder="Please tell more about your request ..." class="form-control" aria-label="Please tell more about your request ...">
        </div>
    </div>
</div>