yeah its hit or miss really, small pieces help. Since i got mine in a big flat block its hard to get solid pieces i just hold it at the mouth for a bit. On palys i just hold it by the mouth and touch the skirt until it notices it, if it opens its mouth i stick the mysis in there and hope it holds. If it holds it it will eat it and i leave it otherwise it just spits it out. Took me some time but its fun to do and watch.
There was a piece of mysis just floating around and my goby kept trying to get it and it eventually landed on my biggest duncan head and it swallowed it up quickly. It was cool to see it happen somewhat naturally without me sticking it in its mouth