Simply take the following steps:
S$300 cashback will be credited to your Card Account within 3 to 5 months from meeting the minimum Qualifying Spend.
You may refer to Clause 6 of the terms and conditions for the list of exclusion.
You may check the merchant category of the posted transactions via DBS digibot:
You may register your interest for the Promotion here. Please note that you are still required to fulfil the spending requirements to be eligible for the S$300 cashback.