How to authenticate AAD user in Identity Server 4?

I have integrated the Azure Active Directory in Identity Server 4 as an external provider.

I want to authenticate Azure Active Directory users from Identity Server 4 by using the APIs.

Here is the code:

var disco = await client.GetDiscoveryDocumentAsync("https://localhost:5001");

if (disco.IsError)
{
    Console.WriteLine(disco.Error);
    return;
}

// request token
var tokenResponse = await client.RequestPasswordTokenAsync(new PasswordTokenRequest
        {
            Address = disco.TokenEndpoint,
            ClientId = "client",
            ClientSecret = "secret",

            Scope = "api1",
            UserName = "ad user name",
            Password = "add user password"
        });
        

When I execute this piece of code, I got an invalid username or password error.

Note: the provided credentials are valid.

Here is my startup.cs file

using IdentityServer4;
using IdentityServer4.EntityFramework.DbContexts;
using IdentityServer4.EntityFramework.Mappers;
using MorpheusIdentityServer.Quickstart.UI;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.IdentityModel.Protocols.OpenIdConnect;
using Microsoft.IdentityModel.Tokens;
using System.Linq;
using System.Reflection;
using System.Security.Cryptography.X509Certificates;
using IdentityServer4.Validation;

namespace MorpheusIdentityServer
{
    public class Startup
    {
        public Startup(IConfiguration configuration, IWebHostEnvironment env)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews();

            var migrationsAssembly = typeof(Startup).GetTypeInfo().Assembly.GetName().Name;
            string connectionString = Configuration.GetSection("ConnectionString").Value;

            var builder = services.AddIdentityServer()
                .AddConfigurationStore(options =>
                {
                    options.ConfigureDbContext = b => b.UseSqlServer(connectionString,
                        sql => sql.MigrationsAssembly(migrationsAssembly));
                })
                .AddOperationalStore(options =>
                {
                    options.ConfigureDbContext = b => b.UseSqlServer(connectionString,
                        sql => sql.MigrationsAssembly(migrationsAssembly));
                })
                .AddProfileService<ProfileService>();

                 

            services.AddAuthentication()
                 .AddOpenIdConnect("aad", "Azure AD", options =>
                 {

                     options.SignInScheme = IdentityServerConstants.ExternalCookieAuthenticationScheme;
                     options.SignOutScheme = IdentityServerConstants.SignoutScheme;

                     options.Authority = Configuration.GetSection("ActiveDirectoryAuthority").Value;
                     options.ClientId = Configuration.GetSection("ActiveDirectoryClientId").Value;
                     options.ResponseType = OpenIdConnectResponseType.IdToken;
                     options.CallbackPath = "/signin-aad";
                     options.SignedOutCallbackPath = "/signout-callback-aad";
                     options.RemoteSignOutPath = "/signout-aad";
                     options.TokenValidationParameters = new TokenValidationParameters
                     {
                         NameClaimType = "name",
                         RoleClaimType = "role",
                         ValidateIssuer = false
                     };

                 });
            services.AddSingleton<IResourceOwnerPasswordValidator, ValidateExternalUser>();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            InitializeDatabase(app);
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseStaticFiles();
            app.UseRouting();

            app.UseIdentityServer();
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapDefaultControllerRoute();
            });
        }

        private void InitializeDatabase(IApplicationBuilder app)
        {
            using (var serviceScope = app.ApplicationServices.GetService<IServiceScopeFactory>().CreateScope())
            {
                serviceScope.ServiceProvider.GetRequiredService<PersistedGrantDbContext>().Database.Migrate();

                var context = serviceScope.ServiceProvider.GetRequiredService<ConfigurationDbContext>();
                context.Database.Migrate();
                if (!context.Clients.Any())
                {
                    foreach (var client in Config.Clients)
                    {
                        context.Clients.Add(client.ToEntity());
                    }
                    context.SaveChanges();
                }

                if (!context.IdentityResources.Any())
                {
                    foreach (var resource in Config.IdentityResources)
                    {
                        context.IdentityResources.Add(resource.ToEntity());
                    }
                    context.SaveChanges();
                }

                if (!context.ApiScopes.Any())
                {
                    foreach (var resource in Config.ApiScopes)
                    {
                        context.ApiScopes.Add(resource.ToEntity());
                    }
                    context.SaveChanges();
                }
            }
        }
    }
}