simplest Way to WaterMark Of TextBox
<Window.Resources>
<Style x:Key="MyWaterMarkStyle" TargetType="{x:Type TextBox}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<Grid>
<Border Background="White" BorderBrush="#FF7D8683" BorderThickness="1"/>
<ScrollViewer x:Name="PART_ContentHost" Margin="5,0,0,0" VerticalAlignment="Center" />
<Label Margin="5,0,0,0" x:Name="WaterMarkLabel" Content="{TemplateBinding Tag}" VerticalAlignment="Center"
Visibility="Collapsed" Foreground="Gray" FontFamily="Arial"/>
</Grid>
<ControlTemplate.Triggers>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="Text" Value=""/>
</MultiTrigger.Conditions>
<Setter Property="Visibility" TargetName="WaterMarkLabel" Value="Visible"/>
</MultiTrigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Foreground" Value="DimGray"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
and add textbox StaticResource style
<TextBox
Style="{StaticResource MyWaterMarkStyle}"
Tag="Search Category"
Grid.Row="0"
Text="{Binding CategorySearch,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
TextSearch.Text="Search Category"
>