//Put this method in your Application class
public static void addMessage(Message message){
    Text username = new Text(message.getUsername());
    username.setFont(Font.font("Verdana", FontWeight.BOLD, 14));
    Text date = new Text(message.getTimestamp());
    date.setFont(Font.font("Verdana", FontWeight.ITALIC, 12));
    date.setFill(Color.GRAY);
    Text message = new Text(message.getMessage());
    date.setFont(Font.font("Courier New", 12));
    mainBox.getChildren().Add(new HBox(username, date, message)); //mainBox being a VBox that stores all your HBoxes.
}