Place this clip right on the reaction beat, after the surprise has registered and before your response line. Follow it with the anime wow sound for a bigger second reaction, or leave it alone when the scene already carries the joke forward.
About this sound
A short voice clip of a sharp 'Kya!' let out in a high, startled pitch. It feels cute, panicky and funny at once, like a tiny jump scare rather than real fear, which makes the reaction land as playful surprise. The sound is voiced by Todoroki Hajime.