Can't get access to a static public class in C#

namespace Quick_Preview
{
    public partial class Form1 : Form
    {
        private GlobalHotkeys.GBH ghk;
        public Form1()
        {
            InitializeComponent();
            ghk = new GlobalHotkeys.GBH(Constants.NOMOD, Keys.Space, this);
        }
    }
}
//Constants is what "does not exists"

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace GlobalHotkeys
{
    public class GBH
    {
        public void GlobalHotkey(int modifier, Keys key, Form form)
        {
            this.modifier = modifier;
            this.key = (int)key;
            this.hWnd = form.Handle;
            id = this.GetHashCode();

        }

        public static class Constants
        {
            //modifiers
            public const int NOMOD = 0x0000;
            public const int ALT = 0x0001;
            public const int CTRL = 0x0002;
            public const int SHIFT = 0x0004;
            public const int WIN = 0x0008;

            //window message id for hotkey
            public const int WM_HOTKEY_MSG_ID = 0x0312;
        }

        private int modifier;
        private int key;
        private IntPtr hWnd;
        private int id;

        [DllImport("user32.dll")]
        private static extern bool RegisterHotKey(IntPtr hWnd, int id, int fsModifiers, int vk);

        [DllImport("user32.dll")]
        private static extern bool UnregisterHotKey(IntPtr hWnd, int id);



        public override int GetHashCode()
        {
            return modifier ^ key ^ hWnd.ToInt32();
        }

        public bool Register()
        {
            return RegisterHotKey(hWnd, id, modifier, key);
        }

        public bool Unregister()
        {
            return UnregisterHotKey(hWnd, id);
        }
    }
}