您要查找的术语是文本框提示
试试这个：

<Window.Resources>
    <VisualBrush x:Key="SearchHint" TileMode="None" Stretch="None" AlignmentX="Left">
        <VisualBrush.Transform>
            <TranslateTransform X="5" Y="0" />
        </VisualBrush.Transform>
        <VisualBrush.Visual>
            <Grid>
                <TextBlock FontStyle="Italic" Foreground="Black" Opacity="0.3" Text="Search …"/>
            </Grid>
        </VisualBrush.Visual>
    </VisualBrush>
</Window.Resources>

<StackPanel>
    <TextBox VerticalContentAlignment="Center"  Height="30">
        <TextBox.Style>
            <Style TargetType="TextBox">
                <Style.Triggers>
                    <Trigger Property="Text" Value="">
                        <Setter Property="Background" Value="{StaticResource SearchHint}"/>
                    </Trigger>
                </Style.Triggers>
            </Style>
        </TextBox.Style>
    </TextBox>
</StackPanel>

结果：