Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace Holding
{
    public class Startup
    {
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            services
                .AddRazorPages()
                .AddRazorPagesOptions(options => {
                    //options.Conventions.AddPageRoute("/index", "{Page=Holding}");
                    //options.Conventions.AddPageRouteModelConvention("", model => { IndexModel });
                    //options.Conventions.AddPageRoute("/index", "Index");
                    options.RootDirectory = "/";
                });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseRouting();

            // https://www.dotnetperls.com/maprazorpages-aspnet-core
            app.UseEndpoints(endpoints =>
            {
                /*endpoints.MapGet("/", async context =>
                {
                    await context.Response.WriteAsync("Hello World!");
                });*/
                endpoints.MapRazorPages();
            });
        }
    }
}