How can I access my TabItems on Usercontrol from MainWindow

Hi all,
 
I would like to ask you, how can I access my usercontrol from MainWindow?
 
For accessing to MainWindow I'm using : 

Dim mw As MainWindow = DirectCast(Application.Current.MainWindow, MainWindow)

I need this because I would like to select particular TabItem from my UserControl. (tabitem.IsSelected= true)
 
Thank you in advance