<!--ROUND TEXTBOX -->
<Style x:Key="roundTextBox" TargetType="{x:Type TextBox}">
    <Setter Property="KeyboardNavigation.TabNavigation" Value="None" />
    <Setter Property="AllowDrop" Value="true" />
    <Setter Property="Background" Value="Transparent"></Setter>
    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
    <Setter Property="VerticalContentAlignment" Value="Stretch" />
    <Setter Property="FontFamily" Value="Segoe UI" />
    <Setter Property="FontSize" Value="12" />
    <Setter Property="Padding" Value="8,5,3,3" />
    <Setter Property="BorderThickness" Value="0" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type TextBox}">
                <Grid>
                    <Border x:Name="BorderBase" Background="White" BorderThickness="1.4,1.4,1,1"
                            BorderBrush="Silver" CornerRadius="10" />
                    <Label x:Name="TextPrompt" Content="{TemplateBinding Tag}" Visibility="Collapsed" Focusable="False"  Foreground="Silver"></Label>
                    <ScrollViewer Margin="0" x:Name="PART_ContentHost" Foreground="{DynamicResource OutsideFontColor}" />
                </Grid>
                <ControlTemplate.Triggers> 
                    <MultiTrigger>
                        <MultiTrigger.Conditions>
                            <Condition Property="IsFocused" Value="False"></Condition>
                            <Condition Property="Text" Value=""></Condition>
                        </MultiTrigger.Conditions>
                        <MultiTrigger.Setters>
                            <Setter Property="Visibility" TargetName="TextPrompt" Value="Visible"></Setter>
                        </MultiTrigger.Setters>
                    </MultiTrigger>
                    <Trigger Property="IsFocused" Value="True">
                        <Setter Property="BorderThickness" TargetName="BorderBase" Value="2.4,2.4,1,1"></Setter>
                    </Trigger>
                    <Trigger Property="IsEnabled" Value="False">
                        <Setter Property="Foreground" Value="DimGray" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

1. I placed a Label control behind the ContentHost of the TextBox and bound it’s content to the ‘Tag’ property.
2. Next we created a MultiTrigger to hide the TextPrompt when the roundTextBox gets focus or we’ve entered text.
3. This means we can set the text of the prompt on each instance of our roundTextBox simply by entering the text into the Tag

<TextBox Tag="Client Name" Style="{StaticResource roundTextBox}" Name="txtNameCriteria" Height="26" HorizontalAlignment="Left" Margin="104,30,0,0" VerticalAlignment="Top" Width="164" />

This is how the roundTextBox’ behave now – when we give a textbox focus the prompt disappears and when it loses focus the prompt reappears.

How Easy is That!!