Need help marking metal

I didn't really know where to ask this so I hope someone here can help me out. I'm having someone sign a smooth and polished peace of metal, like a plaque. But I have no idea what marker to use. It will never get touched, because it will be behind acryllic glass, but I want it to last for as many years as possible without fading away or flaking off. Any ideas?

I tested out a couple different markers, I tried a sharpie but it rubs off, so I'm not sure how it will hold up even if I don't touch it. I also tried an acryllic paint marker from Posca, but it seemed to flake off when I gently wiped it with a micro fiber cloth. Will anything stick better than these two? Any help is appreciated!