Set the WS_EX_TRANSPARENT
flag for the window's extended style. It makes the window transparent to mouse clicks.
public const int WS_EX_TRANSPARENT = 0x00000020;
public const int GWL_EXSTYLE = (-20);
[DllImport("user32.dll")]
public static extern int GetWindowLong(IntPtr hwnd, int index);
[DllImport("user32.dll")]
public static extern int SetWindowLong(IntPtr hwnd, int index, int newStyle);
int extendedStyle = GetWindowLong(hwnd, GWL_EXSTYLE);
WinAPI.SetWindowLong(hwnd, GWL_EXSTYLE, extendedStyle | WS_EX_TRANSPARENT);