I would put it on the AMCAS if you've already started it by the time you submit. Otherwise, there are usually questions on the secondary asking about your research experiences and/or what you are currently doing if you aren't in school. Those are good places to put them. Lastly, bring it up during interviews. But I wouldn't go out of your way to email/call admissions offices with this info and some schools don't accept updates until they invite you for an interview anyway.
But, in the event that you get published, feel free to flaunt it like it's hot.
