try { final Field f1 = this.promptTextField.getClass().getSuperclass().getDeclaredField("promptText"); f1.setAccessible(true); f1.set(this.promptTextField, new SimpleStringProperty("Shift-Enter = Send prompt\nDouble-Click on message = Copy text to clipboard"));

} catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {

    e.printStackTrace();
}