Passing a value from 1 Window to Another

Hi, I have 2 Windows in my WPF C# project.

I have a textbox in both windows. One is called genBox.Text (passGenerator.xaml) and the other is calledpassBox.Password (MainWindow.xaml). I want the user to be able to pass the string from thegenBox to the passBox. Both windows are open at the same time, if that helps.

I have looked everywhere, but nothing seems to work, or is too confusing. 

Any help would be great, and much appreciated! 

Cheers, guys!

Lee.