Sending Mouse Click in C#

// You can't do that by sending messages, instead use mouse_event Windows API, here is the code :

// Call method ClickOnPoint, this is an example from form click event, so this.handle is form handle, note that these are client coordinates on window witch handle is send, you can easily change this and send screen coordinates, and in that case you don't need handle or ClientToScreen call below.

ClickOnPoint(this.Handle, new Point(375, 340));

// Code:
private void ClickOnPoint(IntPtr wndHandle, Point clientPoint)
{
  var oldPos = Cursor.Position;

  /// get screen coordinates
  ClientToScreen(wndHandle, ref clientPoint);

  /// set cursor on coords, and press mouse
  Cursor.Position = new Point(clientPoint.X, clientPoint.Y);
  mouse_event(0x00000002, 0, 0, 0, UIntPtr.Zero); /// left mouse button down
  mouse_event(0x00000004, 0, 0, 0, UIntPtr.Zero); /// left mouse button up

  /// return mouse
  Cursor.Position = oldPos;
}
// Declarations :

    [DllImport("user32.dll")]
    static extern void mouse_event(uint dwFlags, uint dx, uint dy, uint dwData, UIntPtr dwExtraInfo);

    [DllImport("user32.dll")]
    static extern bool ClientToScreen(IntPtr hWnd, ref Point lpPoint);