This promotion is run by Samsung, not Optus so you're best to ask Samsung that question even though you got your phone from Optus. They'll be able to tell you what else you need.
How did you sign up for the phone @Tung Stran
The proof of purchase is your receipt. We should have emailed the documentation to you after having the contract.
Valid proof of purchase.
If you'd like us to generate a POP for you, we can definitely do that.
Otherwise, jump onto Live Chat and they shouldn't have any issues doing this for you.