<Window x:Class="CloseOwnerWin.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:CloseOwnerWin"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Button Content="Окрыть ещё окно" Padding="10"
                VerticalAlignment="Center" HorizontalAlignment="Center" Click="Button_Click"
                />
    </Grid>
</Window>

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            new MainWindow() { Owner = this }.Show();
        }
    }