Program.cs

using Microsoft.Extensions.FileProviders;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllersWithViews();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}

app.UseHttpsRedirection();

var fileProvider = new PhysicalFileProvider(
    Path.Combine(app.Environment.WebRootPath, "Portal.App"));
app.UseDefaultFiles(new DefaultFilesOptions
{
    FileProvider = fileProvider,
    RequestPath = "",
    DefaultFileNames = new[] { "index.html" }
});
app.UseStaticFiles(new StaticFileOptions
{
    FileProvider = fileProvider,
    RequestPath = ""
});

app.UseRouting();

app.MapControllerRoute(
    name: "default",
    pattern: "{controller}/{action=Index}/{id?}");

app.MapFallbackToFile("index.html", new StaticFileOptions
{
    FileProvider = fileProvider,
    RequestPath = ""
});

app.Run();