How can I append custom css file fo ck editor ?

I found this method.
https://docs.ckeditor.com/ckeditor4/latest/guide/plugin_sdk_styles.html

CKEDITOR.plugins.add( 'example', {
init: function( editor ) {
var pluginDirectory = this.path;
editor.addContentsCss( pluginDirectory + 'styles/example.css' );
}
} );

How ?