Windows Phone Silverlight apps had a MarketplaceReviewTask to prompt users to review an app. Windows Phone Runtime apps don't have an explicit task for this but can launch ms-windows-store: URIs to go to the review page.
Use this with care and thought!
Happy users give good reviews. Irritated users give bad ones. An occasional prompt after the user has successfully completed a task will likely get better reviews than frequent prompts which get in the way of getting things done (I admit it: I've filed bad reviews just to make the darn things go away).
Consider prompting after the user beats a friend's high score, completes a lesson with a good grade, or meets a fitness goal.
Avoid prompting when the user does poorly on a game, or initially launches the app: the user opened the app to use it and may be irritated to get pushed off elsewhere.
There isn't a good way to tell if the user actually filed the review. I'd stop asking after sending the user to the review page once.
await Windows.System.Launcher.LaunchUriAsync(new Uri("ms-windows-store:reviewapp));
await Windows.System.Launcher.LaunchUriAsync(new Uri("ms-windows-store:review?PFN=" + packageFamilyName));
We'll update this entry with Windows 10 information when the unified Windows Dev Center is fully up and running.