The Prompt Decorator allows your to place a watermark on any TextBox control to give your users a hint for what to enter. Any UI element can be used as a watermark, not just a TextBlock.
Providing a text prompt to a TextBox in Xaml is as simple as wrapping the TextBox in a PromptDecorator instance, and setting the Prompt property to your desired text. An image, border, path or another UI element is just as easy - place it an a DataTemplate and set the PromptTemplate property to that resource.
This property allows you to specify if you want the prompt's visibility to collapse when the TextBox gains focus, or stay visible and only disappear upon text input.