Overlay over another fullscreen application

Question

I'm working on a program for my brother-in-law. He's a technician setting up live betting stations. He requested a program that overlays their software, displaying the MAC address. It would make things easier for him and his colleagues. They could ask HQ, but it would take at least a year to get the program, and his boss already gave the green light for this if I could make it. So far, I've managed to create the program in C#, but it won't overlay on fullscreen applications. Is there an easy way to achieve this?

using System;
using System.Net.NetworkInformation;
using System.Reflection.Metadata;
using System.Runtime.InteropServices;
using Timer = System.Windows.Forms.Timer;

namespace TransparentOverlay {
    public partial class DisplayMac : Form {
        private Label macLabel;

        public DisplayMac() {
            InitializeComponent();
            InitializeUI();
            ShowMACAddress();
            SetForegroundWindow(this.Handle);
        }

        private void InitializeUI() {
            FormBorderStyle = FormBorderStyle.None;
            BackColor = System.Drawing.Color.Black; // Set the color to the stroke color
            TransparencyKey = System.Drawing.Color.Blue;
            Size = new System.Drawing.Size(200, 30);
            StartPosition = FormStartPosition.Manual;
            Location = new System.Drawing.Point(Screen.PrimaryScreen.WorkingArea.Width - Width, Screen.PrimaryScreen.WorkingArea.Height - Height);

            macLabel = new Label {
                Text = "MAC Address: ",
                Location = new System.Drawing.Point(10, 10),
                AutoSize = true,
                ForeColor = System.Drawing.Color.White,
                BackColor = System.Drawing.Color.Black,
            };

            Controls.Add(macLabel);    // Then add the main label on top
        }

        private void ShowMACAddress() {
            string macAddress = GetMACAddress();
            macLabel.Text += macAddress;
            Timer timer = new Timer {
                Interval = 1000 // Update every 1000 milliseconds (1 second)
            };
            timer.Tick += (sender, e) => macLabel.Text = "MAC Address: " + GetMACAddress();
            timer.Start();

            SetWindowExTransparent(this.Handle);
        }

        private string GetMACAddress() {
            string mac = string.Empty;
            foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces()) {
                if (nic.NetworkInterfaceType != NetworkInterfaceType.Loopback && nic.OperationalStatus == OperationalStatus.Up) {
                    byte[] physicalAddressBytes = nic.GetPhysicalAddress().GetAddressBytes();
                    mac = string.Join(":", physicalAddressBytes.Select(b => b.ToString("X2")));
                    break;
                }
            }
            return mac;
        }

        [DllImport("user32.dll", SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

        private const int WS_EX_LAYERED = 0x80000;
        private const int WS_EX_TRANSPARENT = 0x20;
        private const uint SWP_NOMOVE = 0x2;
        private const uint SWP_NOSIZE = 0x1;
        private static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);

        [DllImport("user32.dll", SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool SetLayeredWindowAttributes(IntPtr hwnd, uint crKey, byte bAlpha, uint dwFlags);

        private const uint LWA_COLORKEY = 0x1;
        private const uint LWA_ALPHA = 0x2;

        private void SetWindowExTransparent(IntPtr handle) {
            SetWindowPos(handle, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
            SetLayeredWindowAttributes(handle, 0, 128, LWA_ALPHA);
            SetWindowLong(handle, GWL_EXSTYLE, GetWindowLong(handle, GWL_EXSTYLE) | WS_EX_LAYERED | WS_EX_TRANSPARENT);
        }

        private const int GWL_EXSTYLE = -20;

        [DllImport("user32.dll", SetLastError = true)]
        static extern int GetWindowLong(IntPtr hWnd, int nIndex);

        [DllImport("user32.dll", SetLastError = true)]
        static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

        [System.Runtime.InteropServices.DllImport("user32.dll")]
        public static extern bool SetForegroundWindow(IntPtr hWnd);


        /// <summary>
        ///  The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main() {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new DisplayMac());
        }
    }
}