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 mb-3">
<div class="col-sm-offset-4 col-sm-8">
<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 mb-3">
<div class="col-sm-offset-4 col-sm-8">
<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>