"How do I get Wellcome/Octopus rewards if the stickers are for the pots? "
You ask for an application form in the store. Fill it in and give it back to store staff. Then it takes about 10 days for your Octopus card to "activate". You can then swipe your card at the Octopus reader when they say "Octopus rewards?" during payment.
I'm pretty sure you can also apply online here:
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Not that the sticker thing is totally unrelated with Octopus Rewards.