I used UOB APP to register, u can click log in, then fill in ur details, then will send u an OTP,  if got previous citibank cc just key in ur citibank cc card number ( Dont copy n paste, u need to key in each number by itself), it will then pop up ur new user name and ask u to create ur new password.
I click create new password....after created then ur directed to a log in page again....key in ur username and new password.

Done