$form['advanced'] = array(
'#type' => 'details',
'#title' => t('Advanced settings'),
'#description' => t('Lorem ipsum.'),
'#open' => TRUE, // Controls the HTML5 'open' attribute. Defaults to FALSE.
);<details open="open" class="form-wrapper" id="edit-advanced">
<summary>Advanced settings</summary>
<div class="details-wrapper">
<div class="details-description">Lorem ipsum.</div>
</div>
</details>