How do I make a service call while overriding a navigation request?

[V0du]

I need access to IAuthenticationService to make sure that the user is logged in before navigating to a certain page.

The following code is based on the doc example here

private static void RegisterRoutes(IViewRegistry views, IRouteRegistry routes)
{
    new ("Login", View: views.FindByViewModel<LoginViewModel>()),
    new ("Main", View: views.FindByViewModel<MainViewModel>(),
        Init: (request) => 
        {
           // Need to check if user is logged in
           IAuthenticationService authService = ....

           return request;
       }),
}

in the Uno App template the previous method is called by the UseNavigation method of IHostBuilder:

var builder = this.CreateBuilder(args)
   .Configure(host => host
   ....
   .UseNavigation(ReactiveViewModelMappings.ViewModelMappings, RegisterRoutes);

How can I get an instance of IAuthenticationService in the Init callback?