[ADDED] the inlog screen and a data singleton for the client

Client/Views/GameWindow.xaml.cs

@@ -18,7 +18,7 @@ namespace Client.Views
     /// </summary>
     public partial class GameWindow : Window
     {
-
+        ClientData data = ClientData.Instance;
         public GameWindow()
         {
             DataContext = new ViewModelGame();
@@ -110,7 +110,7 @@ namespace Client.Views
          */
         private void WriteToChat(string message)
         {
-            string user = "Monkey"; 
+            string user = data.User.Username; 
             SentMessage.AppendText($"{user}: {message}\n");
         }
     }

Client/Views/LoginScreen.xaml

@@ -0,0 +1,29 @@
+<Window x:Class="Client.Views.LoginScreen"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:local="clr-namespace:Client.Views"
+        mc:Ignorable="d"
+        Title="LoginScreen" Height="450" Width="800">
+    <Grid>
+        <Grid.RowDefinitions>
+            <RowDefinition Height="60"/>
+            <RowDefinition Height="50"/>
+            <RowDefinition Height="30"/>
+        </Grid.RowDefinitions>
+
+        <Grid.ColumnDefinitions>
+            <ColumnDefinition />
+            <ColumnDefinition />
+        </Grid.ColumnDefinitions>
+
+        <Label Grid.Row="0" FontSize="40" Content="Welcome to Scrubl.io"/>
+        <Label Grid.Row="1" FontSize="30" Content="Enter a username:"/>
+        <Label Grid.Row="2" FontSize="15" Content="(max amount of characters for the username is 10)"/>
+
+        <TextBox Name="usernameTextbox" Grid.Row="1" Grid.Column="1" MaxLength="10" FontSize="30" VerticalAlignment="Center" HorizontalAlignment="Left" Width="250"/>
+        <Button Content="ENTER" Grid.Column="1" Grid.Row="1" VerticalAlignment="Center" HorizontalAlignment="Right" Width="100" Height="40" Click="Button_EnterUsername"/>
+
+    </Grid>
+</Window>

Client/Views/LoginScreen.xaml.cs

@@ -0,0 +1,40 @@
+using SharedClientServer;
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Data;
+using System.Windows.Documents;
+using System.Windows.Input;
+using System.Windows.Media;
+using System.Windows.Media.Imaging;
+using System.Windows.Shapes;
+
+namespace Client.Views
+{
+    /// <summary>
+    /// Interaction logic for LoginScreen.xaml
+    /// </summary>
+    public partial class LoginScreen : Window
+    {
+        ClientData data = ClientData.Instance;
+        public LoginScreen()
+        {
+            InitializeComponent();
+        }
+
+        private void Button_EnterUsername(object sender, RoutedEventArgs e)
+        {
+            User user = new User(usernameTextbox.Text, 0, false);
+            Client client = new Client();
+
+            data.User = user;
+            data.Client = client;
+
+            MainWindow startWindow = new MainWindow();
+            startWindow.Show();
+            this.Close();
+        }
+    }
+}

Client/Views/MainWindow.xaml.cs

@@ -21,9 +21,13 @@ namespace Client
     /// </summary>
     public partial class MainWindow : Window
     {
+        ClientData data = ClientData.Instance;
         public MainWindow()
         {
+            this.DataContext = new ViewModel();
             InitializeComponent();
+
+            usernameTextbox.Text = data.User.Username;
         }
 
         private void Button_Click(object sender, RoutedEventArgs e)