Think you can send it in through united.com/feedback, believe one of the options is compliment.

You can also send something in through the outperform recognition program on the mobile app or mobile website. However, employee first and last name, as well as last 4 digits of employee number are supposed to be required fields. Did just go in and notice you can scan their badge now and presumably get that pre-filled, but definitely think the employee number requirement is a bit strange.