Looks like there is no implemented functionality for this and you need to do it yourself.
You can get some idea from Callisto WatermarkTextBox
or from the next question and answers - Watermark / hint text TextBox in WPF
문제
How to add text placeholder to PasswordBox control in Windows 8 App while it is without text and doesn't get focus?
해결책 2
Looks like there is no implemented functionality for this and you need to do it yourself.
You can get some idea from Callisto WatermarkTextBox
or from the next question and answers - Watermark / hint text TextBox in WPF
다른 팁
WinRT XAML Toolkit has a WatermarkPasswordBox now. You can get it off of NuGet. Also, for Windows 8.1 you can use the PlaceholderText property.