Watermark/Placeholder on a WPF Combobox control

I can do the watermark text/placeholder on a WPF textbox control by using the below XAML code.

    <TextBox Name="txtFilter" Grid.Row="1" Height="25" Width="250" BorderBrush="Black" BorderThickness="1" HorizontalAlignment="Left" Margin="5,0,0,0" TextChanged="txtFilter_TextChanged">
            <TextBox.Style>
                <Style BasedOn="{StaticResource {x:Type TextBox}}" TargetType="{x:Type TextBox}">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="{x:Type TextBox}">
                                <Border Background="White" CornerRadius="5" BorderBrush="Black" BorderThickness="1">
                                    <Grid>
                                        <ScrollViewer x:Name="PART_ContentHost" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                                        <TextBlock x:Name="textBlock" VerticalAlignment="Center" Opacity="0.5" Text=" Search by Customer Name " Foreground="Blue" FontStyle="Italic" Visibility="Hidden" />
                                    </Grid>
                                </Border>
                                <ControlTemplate.Triggers>
                                    <MultiTrigger>
                                        <MultiTrigger.Conditions>
                                            <Condition Property="IsFocused" Value="False" />
                                            <Condition Property="Text" Value="" />
                                        </MultiTrigger.Conditions>
                                        <Setter Property="Visibility" TargetName="textBlock" Value="Visible" />
                                    </MultiTrigger>
                                </ControlTemplate.Triggers>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </TextBox.Style>
        </TextBox>

But how it is possible for a WPF Combo box control ? I tried with the below shown XAML code. But it is not working.

      <ComboBox.Resources>
                <VisualBrush x:Key="HelpBrush" TileMode="None" Opacity="0.4" Stretch="None" AlignmentX="Left">
                    <VisualBrush.Visual>
                        <TextBlock FontStyle="Italic" Opacity="0.5" Text="Type or select from list" Foreground="Blue" Visibility="Visible"/>
                    </VisualBrush.Visual>
                </VisualBrush>
     </ComboBox.Resources>
            <ComboBox.Style>
                <Style TargetType="ComboBox">
                    <Style.Triggers>
                        <Trigger Property="Text" Value="{x:Null}">
                            <Setter Property="Background" Value="{StaticResource HelpBrush}"/>
                        </Trigger>
                        <Trigger Property="Text" Value="">
                            <Setter Property="Background" Value="{StaticResource HelpBrush}"/>
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </ComboBox.Style>

Can anyone help me to place a watermark text on a WPF Combobox control ???