Startup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using UserRegistrationWebAPI.Models;
using Microsoft.EntityFrameworkCore;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.IdentityModel.Tokens;
using System.Text;

namespace UserRegistrationWebAPI
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            //Inject AppSettings
            services.Configure<ApplicationSettings>(Configuration.GetSection("ApplicationSettings"));

            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

            services.AddDbContext<AuthenticationContext>(options => 
                options.UseSqlServer(Configuration.GetConnectionString("IdentityConnection")));

            services.AddDefaultIdentity<ApplicationUser>()
                .AddRoles<IdentityRole>()
                .AddEntityFrameworkStores<AuthenticationContext>();

            services.Configure<IdentityOptions>(options =>
            {
                options.Password.RequireDigit = false;
                options.Password.RequireNonAlphanumeric= false;
                options.Password.RequireLowercase= false;
                options.Password.RequireUppercase= false;
                options.Password.RequiredLength = 4;
            });

            services.AddCors();

            //JWT Authentication
            var key = Encoding.UTF8.GetBytes(Configuration["ApplicationSettings:JWT_Secret"].ToString());

            services.AddAuthentication(x =>
            {
                x.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
                x.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
                x.DefaultScheme = JwtBearerDefaults.AuthenticationScheme;
            }).AddJwtBearer(x => {
                x.RequireHttpsMetadata = false;
                x.SaveToken = false;
                x.TokenValidationParameters = new TokenValidationParameters
                {
                    ValidateIssuerSigningKey = true,
                    IssuerSigningKey = new SymmetricSecurityKey(key),
                    ValidateIssuer = false,
                    ValidateAudience = false,
                    ClockSkew = TimeSpan.Zero
                };
            });
        }

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

            app.UseCors(builder =>
            builder.WithOrigins(Configuration["ApplicationSettings:Client_URL"].ToString())
            .AllowAnyHeader()
            .AllowAnyMethod()
            .AllowAnyOrigin());

            app.UseAuthentication();

            app.UseMvc();
        }
    }
}