setAssistivePostfix and ASSISTIVE_NOTIFICATION

I have a MenuBar named initMenuBar.

I’d like to add screen reader accessibility to tell a user to use the arrow keys to find the dropdown choices.

Reading the Vaadin API docs, this is how I intuitively thought to add the screen reader informaton, but it does not work.

initMenuBar.setAssistivePostfix(ASSISTIVE_NOTIFICATION, "Use down arrow to navigate to sub menu choices");

Please advise on how to do this. Thank you.