Apple only lets a Siri Shortcut's trigger phrase capture the item name as a follow-up question, not as part of the same sentence - so even saying "Add milk to my shopping list in Oiko" in one breath, Siri still asks "What's the item?" afterwards. Just answer with the name (e.g. "Milk") and it completes.
Siri checks your existing inventory first, regardless of whether the item is currently in stock, sitting on your shopping list, or hasn't been touched in a while. A match updates that item in place and keeps its existing category. What happens for a genuinely new item depends on which phrase you used: "add to my shopping list" creates it immediately (since you need it on your list right away), while the bare "add to Oiko" phrase queues it in your Pending Items for review instead - open the app and you'll be prompted to set a proper category before it becomes a real inventory item.
Open the app and sign in at least once on your device - that's what lets Siri authenticate on your behalf. If Siri says "You're not logged in to Oiko," open the app, confirm you're signed in, and try again.
iOS 16 or later.
For the most reliable experience, we'd recommend turning on Siri Responses → Prefer Spoken Responses and Always Show Siri Captions (Settings → Accessibility → Siri, or Settings → Siri & Search depending on your iOS version) - this keeps Siri consistently speaking its responses aloud, with the on-screen text staying visible too.