Add a placeholder to TinyMCE editor (Wordpress)

functions.php

<?php 

function set_mce_editor_placeholder_question($textarea_html) {
  if ('question' !== get_post_type()) return $textarea_html;

  $placeholder = __( 'Please enter some text as the question...', 'site_domain');

  $textarea_html = preg_replace( '/<textarea/', "<textarea placeholder=\"{$placeholder}\"", $textarea_html );

  return $textarea_html;
}

add_filter( 'the_editor', 'set_mce_editor_placeholder_question' );