IIS virtualpath and UriBuilder

Question

I'm having multiple applications hosted under one site in IIS. Each application have unique VirtualPath, so i have something like this:

1. localhost/Site1
2. localhost/Site2
3. ...
4. locahost/SiteN

What I'm trying to do - to build valid URL using out of the box helpers, like UriBuilder. Problem is that using default constructor like

var builder = new System.UriBuilder("https://mysite/Site1");

will split my address into

1. Host: mysite
2. Path: Site1

Is there any "right approach" or best practices for generating addresses for sites hosted like this?