I'm sorry to disappoint you, but no, you cannot configure your UWP application as a screensaver provider.
Instead, you can offer the user to customize the control of your window on the lock screen or wallpaper image. UWP applications can set these images even when running in the background job with the UserProfilePersonalizationSettings class:
// Lock Screen await UserProfilePersonalizationSettings.Current.TrySetLockScreenImageAsync(StorageFile); // Wallpaper await UserProfilePersonalizationSettings.Current.TrySetWallpaperImageAsync(StorageFile);
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