Как послать сообщение окну с текстбоксом

SendMessageA(thisWindow, WM_COMMAND, ???, ???);

  textBox1_TextChanged(sender, new EventArgs());

[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, StringBuilder lParam);
//If you use '[Out] StringBuilder', initialize the string builder with proper length first.
 
[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, [MarshalAs(UnmanagedType.LPStr)] string lParam);
//Also can add 'ref' or 'out' ahead of 'String lParam', don't use CharSet.Auto since we use MarshalAs(..) in this
//example.
 
//Works for unicode. One can also use CharSet = CharSet.Unicode instead of [MarshalAs(UnmanagedType.LPWStr)] 
[DllImport("user32.dll", EntryPoint = "SendMessageW")]
static extern IntPtr SendMessageW(IntPtr hWnd, UInt32 Msg, IntPtr wParam, [MarshalAs(UnmanagedType.LPWStr)] string lParam);
 
[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 msg, IntPtr wParam, ref RECT lParam);
 
[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 msg, IntPtr wParam, ref POINT lParam);
 
[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, Int32 wParam, Int32 lParam);

[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);

IntPtr thisWindow = FindWindow(null, "ЧужаяФорма");
IntPtr handle = FindWindowEx(thisWindow, IntPtr.Zero, "ЧужойTextBox", null);
SendMessage(handle, WM_SETTEXT, new IntPtr(0), text);