Ok, so you need to check the Push Notifications capability in the iOS project, but also in the Apple Developer Portal for that app.
When you call the NotificationCenter.RequestAuthorization method, if the user allows notifications, the AuthorizationSucceeded event will fire with an object telling you what the user allows. You should call this method each time the app starts up from then on and look at the results because users can change their notification preferences at any time and you may want to deal with things differently.
Next, you’ll want to call NotificationCenter.RegisterForRemoteNotifications to request a token for that particular device. RemoteRegistrationSucceeded will fire if a token was provided. Again, you’ll want to call this each time the app launches because while it probably won’t change, Apple says that it may change. Store that token on your server with an identifier for this device or user’s account.
Once those things have been done, you can use the Xojo Cloud web framework stuff to send a notification to that particular device using the provided token. It will raise the NotificationReceived event to which you’ll either want to process it locally and not show it or return MobileNotifications.PresentationModes.All to allow the system to show the notification anyway.
Here’s a sample project that I gave to another user that has the setup code for remote notifications in it.
Oh, and I should mention that you’ll also need to go to your apple developer account and register for an Apple Push Notifications service (APNs) Key, which will provide you with the Key ID and certificate that you need to be able to send notifications.