This situation came up when I played Kris in the goat league. This was game 1 and I hadn't used any dustshoot before. It was his MP1 and he had 3 cards in hand and I had two s/t set. He flipped his set merchant which added snatch to his hand. When he added, I wrote "when you add" and waited for a while and wrote "uh" before flipping dustshoot. He said he had priority and and wanted to use MST before dustshoot. I knew that technically there was nothing I could do since I should have specifically asked if he wanted to pass priority. I didn't want to do that because it would have made dustshoot too obvious and I thought that saying "when you add" and waiting for a while would imply he passed priority to me. In the end he got to use MST before I could dustshoot but he let me shuffle my backrow.
Is there way to use dustshoot without specifically asking if your opponent wishes to pass priority? Would saying something like "think" when they have 4+ cards before flipping dustshoot be enough?