The chosen widget is my default option for select widget. This snippet shows you how to add the placeholder in a form alter.
/**
* @param $form
* @param FormStateInterface $form_state
* @param $form_id
* Change placeholder value for chosen
*/
function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
if($form['#id'] === 'MYFORMID') {
$form['MYFIELD']['#attributes']['data-placeholder'] = t('Select an option');
}
}