You must be careful to not expose fundamentally unmanaged types like HWND to C# code. The C# compiler will not allow you to pass a value of such a type. The proper interop type here is IntPtr, it can store a handle value. So make your C++/CLI method look like this:
void VideoCapture::SetVideoWindow(IntPtr windowHandle)
{
VideoWindow = (HWND)windowHandle.ToPointer();
}
You can now simply pass panel1.Handle to the method, also of type IntPtr.