It should be under the benefits tab on the members app. You have to scroll down the screen, its the second option from the bottom, titled "Have you claimed your reward yet?".
It eventually leads you to the claim form page, where you select which item you pre-ordered, press claim and fill out form. They will verify your claim and then send you a voucher, it says within about 5 days, where you then select the reward you want.
This is the link
https://20202.samsungpromotions.claims/gb/en/pages/notepreorder/home
