How to move a borderless window

I had to display a warning message sitting on top of all other windows. I created a borderless form and pinvoked a windows API function in my C# code to achieve this.

However, I found that I couldn't move the borderless window using the mouse. The border actually responds to all resize and position messages. To move a borderless window, the following is needed.

Two API methods:
[DllImport("user32.dll")]
public static extern bool ReleaseCapture();

[DllImport("user32.dll")]
public static extern IntPtr SendMessage(IntPtr hWnd, int msg, int wParam, int lParam);

Handle the mouse down event in the borderless window:
private const int WM_NCLBUTTONDOWN = 0xA1;
private const int HT_CAPTION = 0x2;
 private void borderlessWindow_MouseDown(object sender, MouseEventArgs e)
  {
            if (e.Button == MouseButtons.Left)
            {
                ReleaseCapture();
                SendMessage(this.Handle, WM_NCLBUTTONDOWN,  (IntPtr)HT_CAPTION, IntPtr.Zero);
            }
 }