I don't think you can add relays on amethyst right now. But whenever I add a relay on some other client like snort.social it shows up on amethyst as well.
Discussion
Ok cool, I’ll do this and see what happens
and if you add it on snort make sure you tick the read and write boxes. at first I didn't do that and I didn't know why it wasn't working.
You can add relays by clicking on the top right corner star. This puts a pop up window that shows all the current relays. At the bottom is a field to add a new relay. Once you add one, click the post button at the top right.