What assembly to get application properties?

Question

I have example code that looks like this, but I can't figure out where assembly to include to get Properties ?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Configuration;
using System.Reflection;
using System.Windows;
using System.Windows.Forms;
using System.Drawing;

namespace ScreenSizeEntity
{
    class ScreenSize
    {
        public void Resize(Form scrn)
        {
            if (System.Diagnostics.Debugger.IsAttached)
            {
                Normal(scrn);
            }
            else
            {
                Maximise(scrn);
            }
        }
        public void Maximise(Form scrn, bool MaximiseOverride = false)
        {
            scrn.Text = "";
            scrn.FormBorderStyle = FormBorderStyle.None;
            scrn.WindowState = FormWindowState.Maximized;
            scrn.Height = Screen.PrimaryScreen.Bounds.Height;
            scrn.Width = Screen.PrimaryScreen.Bounds.Width;
            if (!MaximiseOverride)
                Properties.Settings.Default.IsMaximised = true;
            Properties.Settings.Default.Save();
            scrn.Invalidate();
            Application.DoEvents();
            scrn.TopMost = true;
            scrn.Activate();
        }

 ...