It's a good idea to enable push notifications on the Citi Bike mobile app to be notified when you've successfully docked your bike and ended your ride. If you ever walk away from a bike that has not been properly docked, you could be liable for the lost/stolen bike fee of $1200 (+ tax) if someone then takes your bike and doesn't return it.
To enable push notifications, you can easily do this the first time you open the app after downloading it by clicking "OK" on the pop-up message. If you've already clicked "Don't Allow" but change your mind, you can opt into push notifications through the settings on your phone. Open up Settings, select Notifications, scroll down until you find Citi Bike, and toggle to the right to "Allow Notifications."