How can we specify the CC email address in the Triggered send definition? I know this is possible if we create the email definition using code.
But is it possible mention that on the UI?
The cc/bcc functionality is - or should be - available in the front-end when you create the Triggered Send Definition.
If you should not see the cc/bbc function yourself in your account, you should drop the Marketing Cloud Support team a quick note asking them to enable this in your account.