wpf write value to config file and read the persisted value

<Window x:Class="WpfApp26.MainWindow"
        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:WpfApp26"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50"/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Button Grid.Row="0" HorizontalAlignment="Left" Content="Load Folder" FontSize="30" Click="LoadFolderCmd" />
        <RichTextBox x:Name="rtb" Grid.Row="1" />
    </Grid>
</Window>


//

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Forms;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Xml.Linq;

namespace WpfApp26
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            
        }

        void ReadValueFromConfig()
        {
            StringBuilder builder = new StringBuilder();
            foreach(string config in ConfigurationManager.AppSettings)
            {
                builder.AppendLine($"key:{config},value:{ConfigurationManager.AppSettings[config]}");
            } 
            rtb.AppendText( builder.ToString() );
        }

        void WriteValueToConfig(int len)
        {
            System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            for(int i=0;i<len;i++)
            {
                config.AppSettings.Settings.Add(Guid.NewGuid().ToString(), $"{DateTime.Now.ToString("O")}_{Guid.NewGuid().ToString()}");

                // Save the changes in App.config file.
                config.Save(ConfigurationSaveMode.Modified);

                // Force a reload of a changed section.
                ConfigurationManager.RefreshSection("appSettings");
            } 
        }

        private void LoadFolderCmd(object sender, RoutedEventArgs e)
        {
            WriteValueToConfig(10);
            ReadValueFromConfig();
        }
    }
}

config.AppSettings.Settings.Add(Guid.NewGuid().ToString(), $"{DateTime.Now.ToString("O")}_{Guid.NewGuid().ToString()}");

                // Save the changes in App.config file.
                config.Save(ConfigurationSaveMode.Modified);

                // Force a reload of a changed section.
                ConfigurationManager.RefreshSection("appSettings");