AppSettings.cs

namespace WINKCrossViewX
{
    public class AppSettings
    {
        public string Forges { get; set; }
        public string APIUsername { get; set; }
        public string APIPassword { get; set; }
        public string RefreshInterval { get; set; }

    }
}

Settings.xaml

<Window x:Class="WINKCrossViewX.Settings"
        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:WINKCrossViewX"
        mc:Ignorable="d"
        d:DataContext="{d:DesignInstance local:SettingsModel, IsDesignTimeCreatable=True}"
        Title="Settings" Height="450" Width="800">

    <Grid ShowGridLines="False">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="20"></RowDefinition>
            <RowDefinition Height="*"></RowDefinition>
        </Grid.RowDefinitions>
        <TextBlock Grid.Row="0" Grid.Column="0" Text="Communication with use TCP (HTTPS) over ports 444 and 443" />
        <Border BorderThickness="0" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="1">
            <StackPanel>
                <DockPanel>
                    <TextBlock Margin="10,10,10,10">
                        WINK Forge's (IP or FQDN):
                        <LineBreak />
                        <Italic>Seperate with CSV</Italic>

                    </TextBlock>
                    <TextBox Text="{Binding Forges}" Margin="10,0,7,10" Height="100" Width="150" TextWrapping="Wrap" AcceptsReturn="True" HorizontalScrollBarVisibility="Disabled" VerticalScrollBarVisibility="Auto" />
                </DockPanel>
                <DockPanel>
                    <TextBlock Text="API Username:" Margin="10,0,0,10" />
                    <TextBox Text="{Binding APIUsername}" Margin="10,0,10,10" Height="20" Width="160" HorizontalAlignment="Right" />
                </DockPanel>
                <DockPanel>
                    <TextBlock Text="API Password:" Margin="10,0,0,10"/>
                    <TextBox Text="{Binding APIPassword}" Margin="10,0,10,10" Height="20" Width="160" HorizontalAlignment="Right" />
                </DockPanel>
                <DockPanel>
                    <TextBlock Text="Refresh Interval (Seconds):" Margin="10,0,0,10"/>
                    <TextBox Text="{Binding RefreshInterval}" Margin="10,0,10,10" PreviewTextInput="NumberValidationTextBox" Height="20" Width="160" HorizontalAlignment="Right" />
                </DockPanel>
                <DockPanel>
                    <Button x:Name="Save" Click="Save_Click"  Content="Save Settings"  Margin="10,0,0,10" Width="140"/>
                    <Button x:Name="Reset" Click="Reset_Click" Content="Reset to Default" Margin="10,0,0,10" Width="140" />

                </DockPanel>
            </StackPanel>
        </Border>
    </Grid>
</Window>

Settings.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
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.Navigation;
using System.Windows.Shapes;

namespace WINKCrossViewX
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class Settings : Window
    {
        private readonly SettingsManager<AppSettings> _settingsManager;
        private readonly SettingsModel _model;

        public Settings()
        {
            _settingsManager = new SettingsManager<AppSettings>("WINKCrossView.json");
            
            var appSettings = _settingsManager.LoadSettings();

            if (appSettings == null)
            {
                appSettings = new AppSettings()
                {
                    Forges = "192.168.50.100,192.168.50.101",
                    APIUsername = "apiuser",
                    APIPassword = "apiuser",
                    RefreshInterval = "3"
                };
            }

            _model = new SettingsModel
            {
                Forges = appSettings.Forges,
                APIUsername = appSettings.APIUsername,
                APIPassword = appSettings.APIPassword,
                RefreshInterval = appSettings.RefreshInterval
            };

            InitializeComponent();

            DataContext = _model;

        }

        private void Reset_Click(object sender, RoutedEventArgs e)
        {
            _model.Forges = "192.168.50.100,192.168.50.101";
            _model.APIUsername = "user";
            _model.APIPassword = "pass";
            _model.RefreshInterval = "3";
            MessageBox.Show("Reset to Default Settings.");
        }

        private void Save_Click(object sender, RoutedEventArgs e)
        {
            var appSettings = new AppSettings
            {
                Forges = _model.Forges,
                APIUsername = _model.APIUsername,
                APIPassword = _model.APIPassword,
                RefreshInterval = _model.RefreshInterval
            };
            _settingsManager.SaveSettings(appSettings);
            MessageBox.Show("Settings Saved.");
        }

        private void NumberValidationTextBox(object sender, TextCompositionEventArgs e)
        {
            Regex regex = new Regex("[^0-9]+");
            e.Handled = regex.IsMatch(e.Text);
        }

    }
}

SettingsManager.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Newtonsoft.Json;

namespace WINKCrossViewX
{
    /// <summary>
    /// Interaction logic for Settings.xaml
    /// </summary>
    /// 
    public class SettingsManager<T> where T : class
    {
        private readonly string _filePath;

        public SettingsManager(string fileName)
        {
            _filePath = GetLocalFilePath(fileName);
        }

        private string GetLocalFilePath(string fileName)
        {
            string appData = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData);

            return System.IO.Path.Combine(appData, fileName);
        }

        public T LoadSettings() =>
            File.Exists(_filePath) ?
                JsonConvert.DeserializeObject<T>(File.ReadAllText(_filePath)) :
                null;

        public void SaveSettings(T settings)
        {
            string json = JsonConvert.SerializeObject(settings);
            File.WriteAllText(_filePath, json);
            //MessageBox.Show("Writing: " + _filePath + "\r\n" + json);
        }
    }
}

SettingsModel.cs

using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace WINKCrossViewX
{
    public class SettingsModel: INotifyPropertyChanged
    {
        private string _APIUsername;
        private string _Forges;
        private string _APIPassword;
        private string _RefreshInterval;

        public event PropertyChangedEventHandler PropertyChanged;
        
        protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        public string Forges
        {
            get { return _Forges; }
            set
            {
                if (value != _Forges)
                {
                    _Forges = value;
                    OnPropertyChanged();
                }
            }
        }

        public string APIUsername
        {
            get { return _APIUsername; }
            set
            {
                if (value != _APIUsername)
                {
                    _APIUsername = value;
                    OnPropertyChanged();
                }
            }
        }

        public string APIPassword
        {
            get { return _APIPassword; }
            set
            {
                if (value != _APIPassword)
                {
                    _APIPassword = value;
                    OnPropertyChanged();
                }
            }
        }

        public string RefreshInterval
        {
            get { return _RefreshInterval; }
            set
            {
                if (value != _RefreshInterval)
                {
                    _RefreshInterval = value;
                    OnPropertyChanged();
                }
            }
        }
    }
}