Postly is currently in beta. Before it's available on the App Store, add it to your home screen to get the full experience — including notifications when your posts are ready.
Tap the ⎙Share button at the bottom of your browser
Scroll down and tap Add to Home Screen
Tap Add in the top right
Then reopen Postly from your home screen to enable notifications.
Use your browser's menu to "Add to Home Screen" or "Install App."