In your DBS online banking account set up a new Funds Transfer to Other Bank.
1. Funds Transfer > To Other Bank
2. Add Account :
a. Payee Bank Code : Citibank NA (7214)
b. Payee Branch Code : 011 (the same for all personal credit cards)
c. Payee Account Number : Type in the LAST 11 numbers (that is, omit the first 5 numbers)
d. Name of Payee : your own choice, I used "CITI VISA *****(my surname + initial)" so I could tell which card I am paying for.