After investigation, I found the Uri:
private async void launch()
{
string launchNokiaMaps = "explore-maps://v1.0/?latlon=56.615495,12.1865081&zoom=5";
await Windows.System.Launcher.LaunchUriAsync(new Uri(launchNokiaMaps));
}
That will lauch the Nokia Maps app.
However you should be careful with that because Nokia Maps is not necessarily installed on Windows Phone 8 devices. For example, HTC devices come with the default "Maps" app.
So if you just want to open the default Maps app, you should use the MapsTask. That will work on every Windows Phone 8 device.