New Post: WatermarkTextBox and decimal value

I want use the WatermarkTextBox and show a decimal value in the format "0.00" or "25.75"

How can I do that with MVVM? If I bind the Text-Property to my ViewModel then it show only a "0"

Here a example of my code

Text="{Binding SelectedArticle.Weight,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"

Thanks for support