<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <GroupBox>
            <GroupBox.Header>
                    <CheckBox x:Name="IsFit" Content="Header" Tag="Test"/>
            </GroupBox.Header>
            <StackPanel>
                <TextBlock Text="{Binding IsChecked, Source={x:Reference IsFit}}"/>
                <TextBlock Text="{Binding IsChecked, ElementName=IsFit}"/>
            </StackPanel>
        </GroupBox>
    </Grid>
</Window>