You can have the window object in Tag
of your Border
and then can access it using PlacementTarget.Tag
<DataTemplate>
<Border Padding="5" CornerRadius="10" BorderThickness="1" BorderBrush="Red"
Tag="{Binding RelativeSource={RelativeSource FindAncestor,
AncestorType=Window}}">
<Border.ContextMenu>
<ContextMenu ItemsSource="{Binding PlacementTarget.Tag.PropertyName,
RelativeSource={RelativeSource Self}}"/>
</Border.ContextMenu>
<TextBlock Text="{Binding}"/>
</Border>
</DataTemplate>