Where the code appears
The registration number is printed on the packaging, usually on the back or side panel alongside the ingredient list and manufacturer details. On a carton it tends to sit near the bottom of the back face, close to the batch and expiry information. On a bottle it is often on the label band rather than the cap.
On online listings, it is often shown in the product images rather than the text, so check the photos carefully. Zoom the back-panel shot if the seller uploaded one. Some sellers list it in the description. If neither the photos nor the description show a code, ask before ordering — and treat a vague answer as information in itself.
Where it hides most often: under a folded flap, on the panel that faces away from the bottle's front, or in the third or fourth listing photo rather than the first. Those are the spots worth opening before you add anything to a cart.