sendkey enter to a pop up on application in c#

I am trying to close this startup pop up in program by sending an enter key programmatically. I have followed some of the examples in this site about creating a class within the same namespace to handel this, but I don't know how to use it in main form. check the code below from the sendkey class. thank you in advance

class winhandler
{
    [DllImport("user32.dll", SetLastError = true)]
    private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint procId);

    [DllImport("user32.dll", SetLastError = true)]
    private static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);

    [DllImport("user32.dll", SetLastError = true)]
    private static extern bool SetForegroundWindow(IntPtr hWnd);

    public const uint WM_SETTEXT = 0x000C;
    public const uint WM_GETTEXT = 0x000D;
    public const uint EM_SETSEL = 0x000000B1;
    public const uint WM_GETTEXTLENGTH = 0x000E;
    public const uint SMTO_ABORTIFHUNG = 0x0002;
    public const int BM_CLICK = 0x00F5;

    string title = "Card ....";
    public void cancelwindows()
    {
        IntPtr hWnd = FindWindowEx(IntPtr.Zero, IntPtr.Zero, null, title);

        uint loginWindowProcId;
        uint loginWindowThreadId = GetWindowThreadProcessId(hWnd, out loginWindowProcId);

        // now I can just use .NET to find the Process for me...
        Process loginWindowProcess = null;

        if (0 != loginWindowProcId)
        {
            loginWindowProcess = Process.GetProcessById((int)loginWindowProcId);
            loginWindowProcess.WaitForInputIdle();
            SetForegroundWindow(hWnd);
            SendKeys.SendWait("{ENTER}"); 
        }
    }
}

I work with a lot of password protected excel sheets and I wrote something similar to type the password for me whenever it is requested.

Here is an example of a form that waits for a window containing "Notepad" to open before typing into it and closing.

This example uses a timer to check for the window every 500 milliseconds. Obviously, if you're expecting the window to appear, you could simply use a while loop until it appears or something similar.

I'm not 100% sure which part it is that you're having trouble with, but please feel free to let me know if this doesn't answer your question :)

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

namespace CloseWindowTest
{
    public partial class Form1 : Form
    {
        [DllImport("user32.dll")]
        static extern IntPtr GetForegroundWindow();

        [DllImport("user32.dll")]
        static extern int GetWindowText(IntPtr hWnd, StringBuilder text, int count);

        private System.Windows.Forms.Timer windowTimer;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            windowTimer = new System.Windows.Forms.Timer();
            windowTimer.Interval = 500;
            windowTimer.Tick += t_Tick;
            windowTimer.Enabled = true;
        }

        private void t_Tick(object sender, EventArgs e)
        {
            if (GetActiveWindowTitle().Contains("Notepad"))
            {
                try
                {
                    windowTimer.Enabled = false;
                    SendKeys.SendWait("Notepad Detected");
                    Environment.Exit(0);
                }
                catch (Exception wtf) { MessageBox.Show(wtf.ToString()); }
            }
        }

        private string GetActiveWindowTitle()
        {
            const int nChars = 256;
            StringBuilder Buff = new StringBuilder(nChars);
            IntPtr handle = GetForegroundWindow();

            if (GetWindowText(handle, Buff, nChars) > 0)
            {
                return Buff.ToString();
            }
            return null;
        }
    }
}