sample from IdentityServer4.Samples\Quickstarts\4_ImplicitFlowAuthenticationWithExternal\src\IdentityServer\

Startup.cs

// Copyright (c) Brock Allen & Dominick Baier. All rights reserved.
// Licensed under the Apache License, Version 2.0. See LICENSE in the project root for license information.


using System;
using IdentityServer4;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.IdentityModel.Tokens;

namespace IdentityServer
{
    public class Startup
    {
        public IHostingEnvironment Environment { get; }

        public Startup(IHostingEnvironment environment)
        {
            Environment = environment;
        }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc().SetCompatibilityVersion(Microsoft.AspNetCore.Mvc.CompatibilityVersion.Version_2_1);

            var builder = services.AddIdentityServer()
                .AddInMemoryIdentityResources(Config.GetIdentityResources())
                .AddInMemoryApiResources(Config.GetApis())
                .AddInMemoryClients(Config.GetClients())
                .AddTestUsers(Config.GetUsers());

            if (Environment.IsDevelopment())
            {
                builder.AddDeveloperSigningCredential();
            }
            else
            {
                throw new Exception("need to configure key material");
            }

            services.AddAuthentication()
                /*.AddGoogle("Google", options =>
                {
                    options.SignInScheme = IdentityServerConstants.ExternalCookieAuthenticationScheme;

                    options.ClientId = "<insert here>";
                    options.ClientSecret = "<insert here>";
                })    */
                .AddOpenIdConnect("oidc", "OpenID Connect", options =>
                {
                    options.SignInScheme = IdentityServerConstants.ExternalCookieAuthenticationScheme;
                    options.SignOutScheme = IdentityServerConstants.SignoutScheme;
                    options.SaveTokens = true;

                    options.Authority = "https://demo.identityserver.io/";
                    options.ClientId = "implicit";

                    options.TokenValidationParameters = new TokenValidationParameters
                    {
                        NameClaimType = "name",
                        RoleClaimType = "role"
                    };
                });
        }

        public void Configure(IApplicationBuilder app)
        {
            if (Environment.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseStaticFiles();

            app.UseIdentityServer();

            app.UseMvcWithDefaultRoute();
        }
    }
}

this is sample code I've downloaded from the IdentityServer4.Samples github project. However this project is not accessible any more. So I use this gist to share sample code and ask a question on StackOverflow.