It's been a while since I claimed but it was straight forward, Cancel your subscription in disney before it ends to ensure you are not charged for the following month when you trial is due to end.
Open the boost section and claim your code (best doing this when you current subscription ends) under the generated code there should be a use/claim button or something similier, clicking this will take you to the disney promo page where you either sign in or use your email to verify (sorry I can't remember specifically).
Your trial period will then begin again.
At one point generating the code made it expire early, within the month it was generated. I don't know if this is still the case but be sure to check amy expiry updates or play it safe and only generate the code on the month your current subscription ends.
Below is from the help section.
Hope this helps. All the best for '23.
What if I already have Disney+?
You can still take advantage of the subscription from Samsung by managing your existing Disney+ account: 1 – Generate your code to ensure you don’t miss out (you have until 31st March 2023 to use it and it stays here on Boost waiting for you). 2 – Follow the steps here to manage your subscription https://help.disneyplus.com/csp
. Note: Do not delete your Disney+ account to keep your settings etc. 3 – Once your existing Disney+ subscription has ended, you can use your Samsung code as a returning Disney+ customer