This works for us: <base href="/app/" />

Note the trailing slash, it is important.

It is documented in the official Blazor docs here: https://learn.microsoft.com/en-us/aspnet/core/blazor/host-and-deploy/?view=aspnetcore-3.1&tabs=visual-studio#app-base-path

In _host.chtml set base to

<base href="~/" />

Don't add app.UsePathBase("/app") in your pipeline

In IIS site menu Add Application. Bind it to directory where your app exists.

Set up new application pool for that SubApp ( No managed code )

To share Identity Cookie add

services.AddDataProtection()
.PersistKeysToFileSystem(new DirectoryInfo(@"D:\tmp\Keys"))
.ProtectKeysWithDpapi(protectToLocalMachine:true)
.SetApplicationName("SharedCookieApp");

services.ConfigureApplicationCookie(options =>
          {
              options.Cookie.Path = "/";

          });

This is verified to work with .NET 8 in 2024.

Blazor serverside

You need this in Program.cs:

// Important, needs to be directly after .Build() function
app.MapBlazorHub("/subdir");
app.Map("/subdir/signalr", subapp =>
{
  subapp.UsePathBase("/subdir/signalr");
  subapp.UseRouting();
  subapp.UseEndpoints(endpoints => endpoints.MapBlazorHub());
});

Webassembly & Blazor serverside

In App.razor you need to modify the base href:

<base href="/subdir/">

If you want to do this only for production, you can programmatically change it like this:

@inject NavigationManager nav
@if (nav.Uri.Contains("/subdir"))
{
    <base href="/subdir/">
}
else
{
    <base href="/">
}

Then verify that no links, or embedded javascript or CSS starts with a slash. It should look like this:

<script src="_framework/blazor.web.js"></script>
<link href="css/site.min.css" rel="stylesheet" />
<a href="aboutus">About us</a>
<a href="?getUser=1">Get user</a>

Read more details in the official documentation,

Does your sub app share authentication and so on with the primary app? If not, you don't need to do ANY configuration in Blazor at all-- just set up your app in IIS. My homepage is an old WebForms app, with separately-running Blazor apps.

--edit-- DimaSUN has it right. Your various apps and sub-apps don't even need to be the same type.