Textarea preprocess

/**
* Implements template_preprocess_textarea().
*/
function EXAMPLE_preprocess_textarea(&$variables) {
 if (isset($variables['attributes']['id'])) {
   if ($variables['attributes']['id'] == 'edit-customer-profiles-0-entity-field-text-to-support-0-value') {
     $variables['attributes']['placeholder'] = 'Custom placeholder';
   }
 }
}