Check out example codes for "page parent wpf". It will help you in understanding the concepts better.

Code Example 1

private void Button_Click_Upload(object sender, RoutedEventArgs e)
{
    ((w)App.Current.MainWindow).textBox_1.Text = "Your Text";
    //or 
    ((w)Window.GetWindow(this)).textBox_1.Text = "Your Text";
}

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