You do this the same way you do in Xamarin, as you do in an Objective-C or Java application. You use the same API to register the background service.
There are also documents about this for iOS and for Android .
Xamarin uses the same native API for each system as any other native application. That way, everything you want to do with Xamarin can be done just as you would use original tools for vendors. The only difference is the programming language.
Xamarin.Forms is completely irrelevant here. This is an abstraction layer for native user interface interfaces that allow developers to once define a user interface and launch a truly native interface for Android, iOS, Windows Phone, and Windows Store apps. This has nothing to do with the background here.
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