Hi! You’re right that there is no DM function on AO3, but there are still ways to communicate. The best one by far is just to be an enthusiastic reader who takes the time to write meaningful comments. If you’re lucky, the author will respond and it might lead to a conversation! Authors also like to see repeat commenters, so if you end up reading multiple stories of theirs and enjoying them, don’t be afraid to comment! Some people worry about “spamming” kudos and comments, but I can assure you that nothing is more exciting for a writer than to get these. It’s not weird at all, just very flattering!

Beyond the AO3 comment section though, you can always follow your favorite writers on other social media (like Twitter or Tumblr) and interact with them there. If they have any they want readers to check them out on, they’ll usually have them linked in the “profile” section of their AO3 page, or at the bottom of their fic. If your username is consistent across AO3 and social media platforms, they’ll likely recognize you as a commenter and be more motivated to interact or even follow back. At this point, it wouldn’t be too presumptuous to DM and try to start a conversation. I’ve had it where someone DMed me to mention just how much they liked one of my works, and I think that’s a pretty safe way to start a conversation with a writer when feeling socially awkward or even intimidated. I know it’s worked when people have done it with me!

Hope this was helpful and good luck! I learned most of this by accident but it has helped a lot and I hope it can help you 😊