How to update window?

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C#

WPF

Im creating a diary program (for school project) and it consists of a main window with a tab control (3 tabs for different calendar, weekly and list views). The main window has a "New Task" Button which opens a task entry window. On the task entry window, tasks can be saved via a button. I would like the main window to update to include the newly saved task, but am unsure what to include to do this.

Simplification of current setup:

public class MainWindow : Window
{
    public MainWindow()
    {
        //Grid containing tasks
        Button newTask = new Button();
        Button.Click += delegate{new TaskWindow(); };
    }
}

public class TaskWindow : Window
{
    public TaskWindow()
    {
        //Data entry
        Button save = new Button();
        //Save() = function to save
        Button.Click += delegate{Save()};
    }
}