Caliburn Micro: Force user input

I want to force the user to enter some text into a TextBox before he can do anything else. As described in my previous post the TextBox will get the focus when it becomes visible. Now I have to prevent

Caliburn Micro: Focus TextBox when View gets visible

Currently I'm using the WPF framework Caliburn Micro for my UI. With Conductors you can manage the lifecycle of views. If an item gets activated/visible I'd like to focus the TextBox to enhance the user experience. Therefor I've created