Use the feedback site. Use the name of the FA, and if you don't have it, whatever detail you have (the purser on flight XXX on date, the agent who checked me in, the gate agent handling flight YYY on date) - UA definitely can look up that info.

Unfortunately, UA doesn't have an easy process to do so anymore. A long time ago (think it was pre-merger...like that long ago) elites used to get little paper certificates they could fill out and give to the employee in question (I guess similar to DL). Presumably, the employee could give it to their supervisor and got put in their files and noted on their records. Another effort, IIRC, it didn't last longer than about a year, was a system they had where you could submit a compliment through the app where you could submit flight details, employee name (and oddly enough, employee number), and comments, and both the submitter and the employee would be entered into a drawing that they had about every quarter. Can't remember exactly, I think something like 10 customers a quarter were selected to win some amount of MP miles, and they selected a few employees a quarter who also got some reward - not sure what. I feel like that process was around 2013 or so?

But nothing since then, except the feedback site. It's a shame, really - they should make it easier to let them know employees who are doing good. Maybe since they have been working on improving various parts of the experince, they will implement a program for this in the not too far future.