Getting a stamp
At the counter, open the Stamp tab of the Oktapus app. If the shop has a contactless card, bring your phone close to the card: the stamp lands. Otherwise, show the code on screen, and the merchant scans it with their app. That code changes every thirty seconds and serves only once. You have nothing to scan yourself. The poster in the window is a different thing: it enrols you at the shop, it does not give a stamp.