Even if they do have access to the cover letter, someone reviewing dozens of applications is unlikely to go read one in case it has the answer to a question in the application that was left blank.
At a minimum, copy-paste the relevant paragraph from the cover letter, even do not have any more detail to add.
No question in an application form should go unanswered (unless it's innappropiate). It is not unusual for there to be redundancy in all sorts of communication, and as has been mentioned already, the answer to the application may be read by someone who is not provided the cover letter.
I also would be hesitant to simply copy what is in your cover letter. ...