How to send mouse clicks to another application

public partial class B : Form
    {
        const int WM_COMMAND = 0x111;
        const int WM_KEYDOWN = 0x100;
        const int WM_KEYUP = 0x101;


        const int VK_X = 0x58;

        [DllImport("user32.dll")]
        public static extern void SetWindowText(int h, String s);
        [DllImport("user32.dll")]
        private static extern int FindWindow(string ClassName, string WindowName);
        [DllImport("user32.dll")]
        public static extern int SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);

        [DllImport("user32.dll", EntryPoint = "PostMessageA", SetLastError = true)]
        private static extern bool PostMessage(IntPtr hwnd, int Msg, long wParam, long lParam);

        public B()
        {
            InitializeComponent();
        }

        private void btnStart_Click(object sender, EventArgs e)
        {

          int hWnd  = FindWindow(null, "1.txt - Notepad");
            
            if (hWnd > 0)
            {
                IntPtr intptr = new IntPtr(hWnd);
                IntPtr intptrNULL = new IntPtr();
                IntPtr hWndC = FindWindowEx(intptr, intptrNULL, "Edit", "");
                bool result = PostMessage(hWndC, WM_CHAR, VK_X, 0);
            }


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