HeadingTitleConfig

ClassicEditor
  .create( {
	attachTo: document.querySelector( '#editor' ),
	plugins: [ Title, ... ],
	title: {
	  placeholder: 'My custom placeholder for the title'
	},
	root: {
	  placeholder: 'My custom placeholder for the body'
	}
  } )
  .then( ... )
  .catch( ... );