Не получается нажать кнопку в другом приложении

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;
using System.Threading;
using System.Diagnostics;
namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        [DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
        static extern IntPtr FindWindow(IntPtr ZeroOnly, string lpWindowName);
        [DllImport("user32.dll", EntryPoint = "FindWindowEx",  CharSet = CharSet.Auto)]
        static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);
        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, UInt32 wParam, string lParam);
        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, UInt32 wParam, IntPtr buttonHWND);
        [DllImport("user32.dll")]
        private static extern bool PostMessage(IntPtr hWnd, int msg, IntPtr wParam, string lParam);
        
        static uint WM_COMMAND = 0x0111;
        static uint BN_CLICKED = 0x0;
        static uint WM_LBUTTONDOWN = 0x0201;
        static int BM_CLICK = 0x00F5;
 
        public Form1()
        {
            InitializeComponent();
        }
 
        static List<string> GetAllChildrenWindowHandles(IntPtr hParent,  int maxCount)
        {
            List<string> result = new List<string>();
            int ct = 0;
            IntPtr prevChild = IntPtr.Zero;
            IntPtr currChild = IntPtr.Zero;
            while (true && ct < maxCount)
            {
                currChild = FindWindowEx(hParent, prevChild, "Button", null);
                if (currChild == IntPtr.Zero) break;
                result.Add(currChild.ToString());
                prevChild = currChild;
                ++ct;
            }
            return result;
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            IntPtr appHandle = FindWindow(IntPtr.Zero, "Login Error");
            List<string> children = GetAllChildrenWindowHandles(appHandle, 100);
            
            for (int i = 0; i < children.Count; ++i)
                listBox1.Items.Add(children[i].ToString());
 
            SendMessage(appHandle, BM_CLICK,WM_LBUTTONDOWN, children[0].ToString());
        }
 
 
    }
}