NETCore 3.1

AppDbConectx.cs

using IdentityServer.Model;
using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore;

namespace IdentityServer.Data
{
    public class AppDbContext : IdentityDbContext<ApplicationUser>
    {
        public AppDbContext(DbContextOptions<AppDbContext> options)
            : base(options)
        {
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            base.OnConfiguring(optionsBuilder);
        }
    }

}

ApplicationUser.cs

using Microsoft.AspNetCore.Identity;

namespace IdentityServer.Model
{
    public class ApplicationUser : IdentityUser
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public bool IsActive { get; set; }
    }
}

RegisterInStartup.cs

        public void ConfigureServices(IServiceCollection services)
        {

            services.AddDbContext<AppDbContext>(config =>
            {
                config.UseMySQL(_config.GetConnectionString("Default"), sql => sql
                        .CharSetBehavior(CharSetBehavior.NeverAppend))));
            });
            services.AddIdentity<ApplicationUser, IdentityRole>(options => options.SignIn.RequireConfirmedAccount = false)
                .AddEntityFrameworkStores<AppDbContext>()
                .AddDefaultTokenProviders();
        }