What is the minimum number of steps required to display a tooltip when the following control gets focus?

<TextBox ToolTip="Hello there!" ... />

I tried the following in GotFocus

    private void ..._GotFocus(object sender, RoutedEventArgs e) {
        var element = (FrameworkElement)sender;
        var tooltip = element.ToolTip;
        if (!(tooltip is ToolTip)) {
            tooltip = new ToolTip { Content = tooltip };
            element.ToolTip = tooltip;
        }

        ((ToolTip)tooltip).IsOpen = true;
    }

However, it seems to ignore the ToolTipService.Placement for this control and SystemParameters.ToolTipPopupAnimationKey set up level higher.

How can I make it work and honor all settings that generally work for tooltips (except the timing, obviously)?