Best way to give each customer a custom vanity URL - asp.net-mvc

Question

I allow each customer to choose their own URL, by baking their own custom code into the url as the first segment after the domain like this (https://www.[mydomain].com/customercode) and then learn who they are from that URL. This is working fine by learning who they are using MVC routing and session. The problem is that once the customer begins their session using their custom URL, the URL changes and they no longer see their name in the URL. i.e. they start the session as https://www.[mydomain].com/customercode, then the URL changes to something like https://www.[mydomain].com/account/login after the first re-direct.
Is there a way for me to preserve that customer info segment of the URL and just consider it as part of 'the base url'?
In the past I have written extensions such as CustomActionLink and CustomAction functions to override standard url rendering but that would mean that I need to use those functions all over my site. I am wondering if I manipulate or dictate every url rendered by my site, so that the customer name always appears in their url bar.
For reference, here is my routing code
// https://scheduler.[theservicedomain].com/TheCustomersName
routes.MapRoute(
name: "Scheduler",
url: "{practiceRouteCode}",
defaults: new { controller = "Scheduler", action = "Index" }
);
// https://scheduler.theservicedomain].com/scheduler/selectservice
routes.MapRoute(
name: "Default",
url: "{controller}/{action}",
defaults: new { controller = "Scheduler", action = "Index" },
namespaces: new[] { "Scheduler.Controllers" }
);
//https://scheduler.theservicedomain].com/TheCustomersName/scheduler/selectdate
routes.MapRoute(
name: "VanityURL",
url: "{practiceRouteCode}/{controller}/{action}",
defaults: new { controller = "Scheduler", action = "Index" },
namespaces: new[] { "Scheduler.Controllers" }
);
The customer is always required to begin the session with the first or third route, then we know who they are. But as of now the customer information (practiceRouteCode) is dropped and the rest of the session is on the second (Default) route.