using System.Runtime.InteropServices;
using System.Threading;

[DllImport("user32.dll")]
static extern IntPtr FindWindow(IntPtr classname, string title); // extern method: FindWindow
 
[DllImport("user32.dll")]
static extern void MoveWindow(IntPtr hwnd, int X, int Y, int nWidth, int nHeight, bool rePaint); // extern method: MoveWindow
 
[DllImport("user32.dll")]
static extern bool GetWindowRect(IntPtr hwnd, out Rectangle rect); // extern method: GetWindowRect

void FindAndMoveMsgBox(int x, int y, bool repaint, string title)
{
  Thread thr = new Thread(() => // create a new thread
  {
    IntPtr msgBox = IntPtr.Zero;
    // while there's no MessageBox, FindWindow returns IntPtr.Zero
    while ((msgBox = FindWindow(IntPtr.Zero, title)) == IntPtr.Zero) ;
    // after the while loop, msgBox is the handle of your MessageBox
    Rectangle r = new Rectangle();
    GetWindowRect(msgBox, out r); // Gets the rectangle of the message box
    MoveWindow(msgBox /* handle of the message box */, x , y,
      r.Width - r.X /* width of originally message box */,
      r.Height - r.Y /* height of originally message box */,
      repaint /* if true, the message box repaints */);
  });
  thr.Start(); /: starts the thread
}

 
FindAndMoveMsgBox(0,0,true,"Title");
MessageBox.Show("Message","Title");