Ask in person, ask online, and constantly remind people. Sadly when someone has a good experience, it’s not a knee jerk reaction to go post a review whereas you have a bad experience that is the case.
Straight up ask people in the moment at a resident event, anytime they come into the office or you see them around (if things are good), or right when they move in (frame it like how was it working with us during this leasing process).
Digitally, when you wrap up a good email thread with a resident ask them to review and send them direct links, remind them on social media monthly, and make sure links to review sites are in your email signature.