Binding from header to dependency property

<extensions:CustomDataGridTextColumn HeaderStyle="{StaticResource Nagłówek2tb}" Header="Nagłówek 1" MyProperty ="Nagłówek2"/>

Style:
<Style TargetType="{x:Type DataGridColumnHeader}" x:Key="Nagłówek2tb">
            <Setter Property="ContentTemplate">
                <Setter.Value>
                    <DataTemplate >
                        <Grid>
                            <Grid.RowDefinitions>
                                <RowDefinition/>
                                <RowDefinition/>
                            </Grid.RowDefinitions>
                            <TextBlock Grid.Row="0" Text="{Binding}" TextWrapping="Wrap"/>
                            <TextBlock Grid.Row="1" Text="{Binding MyProperty, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=DataGridTextColumn}}"/>
                        </Grid>
                    </DataTemplate>
                </Setter.Value>
            </Setter>
        </Style>

public class CustomDataGridTextColumn : System.Windows.Controls.DataGridTextColumn
    {
 
        public string MyProperty
        {
            get { return (string)GetValue(MyPropertyProperty); }
            set { SetValue(MyPropertyProperty, value); }
        }
 
        public static readonly DependencyProperty MyPropertyProperty =
          DependencyProperty.Register("MyProperty", typeof(string), typeof(CustomDataGridTextColumn), new UIPropertyMetadata(""));
 
    }