I agree with what Alexander said, the proper way to do this would be with URL Routing. But... If you are trying to save time...
First, remove all of your web applications;
So get rid of...
http://myApplication.net/DemoTenant1/
http://myApplication.net/DemoTenant2/
http://myApplication.net/DemoTenant3/
And then you need to make sure that typing in the following:
http://myApplication.net/
... takes you to the actual WebApplication you want to use.
Then, in the global.asax file... you need to capture 404 exceptions.
So when someone types in:
http://myApplication.net/DemoTenant1/
... it will throw a 404 exception which you could catch in your global.asax file like this:
void Application_Error(object sender, EventArgs e)
{
string urlData = Request.ServerVariables["SCRIPT_NAME"];
// do some string splitting to get the DemoTenant1 value
// Response.Redirect("~Login.aspx?tenant=DemoTenant1");
}
Its a bit messy but I have done this in the past when I was in exactly the same situation as you. Although, you do now have the routing module built by Microsoft (which I did not have at the time). I am quite sure that you can use the Routing modules within Webforms, without having to use MVC.