I’ve implemented the Search Contract in my Windows 8 App.
However, when the user brings up the search charm from within the App, the search box is initially blank. I notice that other Apps have a default ‘watermark’ within the box, such as the Bing Maps app:

How can I reproduce this behaviour and add my own watermark?
Okay, it’s not documented on the main Quickstart page or search guidelines, but you do it like this:
I suggest placing it into the OnWindowCreated(…) method in App.xaml.cs