[C#][Help]Postmessage Api

[Bman]

        private void button1_Click(object sender, EventArgs e)
        {

            const int WM_KEYDOWN = 0x100;
            const int WM_KEYUP = 0x101;

            Form1 Frm = new Form1();

            //IntPtr notepad = FindWindow("Form1", null);

            IntPtr editx = FindWindowEx(this.Handle, IntPtr.Zero, "textbox", null);

            PostMessage(editx, WM_KEYDOWN, (int)Keys.A, 0);
            PostMessage(editx, WM_KEYUP, (int)Keys.A, 0);


        }

Im not quite sure why this wont work.

Im trying to send the Key "A" to a textbox on the form that the button is pressed on.
Also, where the code is commented, What would I lock onto? Form1? Form 1's text?

Thanks in advance