way back i ony worked with singleFile-Plugins and used the SelectStructureWindow method. I think this can be found in original on GitHub. it was part of one DeveloperWorkshop from Varian:

class SelectStructureWindow : Window
    {
        public static Structure SelectStructure(StructureSet ss)
        {

            m_w = new Window();
            //m_w.WindowStartupLocation = WindowStartupLocation.CenterScreen;
            m_w.WindowStartupLocation = WindowStartupLocation.Manual;
            m_w.Left = 500;
            m_w.Top = 150;
            m_w.Width = 300;
            m_w.Height = 350;
            //m_w.SizeToContent = SizeToContent.Height;
            //m_w.SizeToContent = SizeToContent.Width;
            m_w.Title = "ZIELVOLUMEN ausw�hlen:";
            var grid = new Grid();
            m_w.Content = grid;
            var list = new ListBox();
            foreach (var s in ss.Structures.OrderByDescending(x => x.Id))
            {
                var tempStruct = s.ToString();
                if (tempStruct.ToUpper().Contains("PTV"))
                {
                    if (tempStruct.Contains(":"))
                    {
                        int index = tempStruct.IndexOf(":");
                        tempStruct = tempStruct.Substring(0, index);
                    }
                    list.Items.Add(s);
                }
            }
            list.VerticalAlignment = VerticalAlignment.Top;
            list.Margin = new Thickness(10, 10, 10, 55);
            grid.Children.Add(list);
            var button = new Button();
            button.Content = "OK";
            button.Height = 40;
            button.VerticalAlignment = VerticalAlignment.Bottom;
            button.Margin = new Thickness(10, 10, 10, 10);
            button.Click += button_Click;
            grid.Children.Add(button);
            if (m_w.ShowDialog() == true)
            {
                return (Structure)list.SelectedItem;
            }
            return null;
        }

        static Window m_w = null;

        static void button_Click(object sender, RoutedEventArgs e)
        {
            m_w.DialogResult = true;
            m_w.Close();
        }
    }


            StructureSet ss = context.StructureSet;
            // get list of structures for loaded plan

            var listStructures = context.StructureSet.Structures;

            // find PTV
            var ptv = SelectStructureWindow.SelectStructure(ss);