window نیست، بلکه کنترلها رو به تب ها اضافه کردند.
<TabControl x:Name="tabControl" HorizontalAlignment="Left" Height="140" Margin="79,60,0,0" VerticalAlignment="Top" Width="336">
<TabItem Header="TabItem">
<Grid Background="#FFE5E5E5">
<TextBlock x:Name="textBlock1" HorizontalAlignment="Left" Margin="60,37,0,0" TextWrapping="Wrap" Text="ExampleTextBlock1" VerticalAlignment="Top" Grid.ColumnSpan="3"/>
</Grid>
</TabItem>
<TabItem Header="TabItem">
<Grid Background="#FFE5E5E5">
<TextBlock x:Name="textBlock2" HorizontalAlignment="Left" Margin="64,46,0,0" TextWrapping="Wrap" Text="ExampleTextBlock2" VerticalAlignment="Top"/>
</Grid>
</TabItem>
</TabControl>