added ApiClient

DDApp/DDApplication/MainWindow.axaml.cs

@@ -1,9 +1,11 @@
+using System;
 using Avalonia.Controls;
 
 namespace DDApplication;
 
 public partial class MainWindow : Window
 {
+    private readonly string _apiKey = Program.Configuration["APIKEY"] ?? throw new ArgumentNullException(nameof(_apiKey));
     public MainWindow()
     {
         InitializeComponent();