Show about window on click

App.axaml.cs

@@ -1,5 +1,7 @@
+using System;
 using Avalonia;
 using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Interactivity;
 using Avalonia.Markup.Xaml;
 using ReceiptPDFBuilder;
 
@@ -21,4 +23,9 @@ public partial class App : Application
 
         base.OnFrameworkInitializationCompleted();
     }
+
+    public void AboutOnClick(object? sender, EventArgs args)
+    {
+        new AboutWindow().Show();
+    }
 }