Put it at the peak of a reaction sequence, after an Oh My God Wow build-up and just before the payoff lands, then let a Nelson Ha Ha follow it to close the beat with a laugh. Keep it as the single punctuation in that beat, with other reactions around it so the sequence builds instead of competing.
About this sound
Iyey is a brief vocal exclamation of about a second and a half, delivered as a sharp, upbeat shout rather than a full sentence. It feels playful and suddenly excited, like a burst of approval or surprise that lifts a moment without slowing it down. Its open, ambiguous tone lets the scene around it decide whether it reads as delight, shock, or comic awkwardness.