Have a look at this for a watermark
Watermark / hint text TextBox in WPF
Basically add a text block that sits over you textbox and then hide it when you dont want the watermark shown anymore.
If you want the custom text, create a dependency property and bind that to the Text
property of the textblock. This way, the user can specify whatever text they want.
public string WaterMark
{
get { return (string )this.GetValue(WaterMarkProperty); }
set { this.SetValue(WaterMarkProperty, value); }
}
public static readonly DependencyProperty WaterMarkProperty = DependencyProperty.Register(
"WaterMark", typeof(string ), typeof(PasswordBoxWin8));
Then you bind to it in the XAML
<TextBlock Text="{Binding WaterMark, ElementName=YourUserControlName}" />
THis way, your user control has a property called WaterMark
that you can set