how to implement JWT token using .net core 8 web api

Please try the followings:

(1) Create Web API project with Authentication type: None.

(2) Install NuGet package Microsoft.AspNetCore.Authentication.JwtBearer.

(3) Register JWT authentication scheme in Program.cs.

using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.IdentityModel.Tokens;
using System.Text;

namespace WebApi
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var builder = WebApplication.CreateBuilder(args);

            // Register JWT authentication scheme
            builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
                .AddJwtBearer(options =>
                {
                    options.TokenValidationParameters = new TokenValidationParameters
                    {
                        ValidateIssuer = true,
                        ValidateAudience = true,
                        ValidateLifetime = true,
                        ValidateIssuerSigningKey = true,
                        ValidIssuer = builder.Configuration["Jwt:Issuer"],
                        ValidAudience = builder.Configuration["Jwt:Issuer"],
                        IssuerSigningKey = new SymmetricSecurityKey(
                            Encoding.UTF8.GetBytes(builder.Configuration["Jwt:Key"]!))
                    };
                });

            // other code (omitted)
        }
    }
}

(4) Add Key and Issuer to appsettings.json

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*",
  "Jwt": {
    "Key": "veryVerySecretKeyWhichMustBeLongerThan32",
    "Issuer": "https://localhost:44362"
  }
}

(5) Add API which verifies username and password sent from user and returns JWT.

using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using Microsoft.IdentityModel.Tokens;
using System.IdentityModel.Tokens.Jwt;
using System.Text;

namespace WebApi.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class TokenController : ControllerBase
    {
        private readonly IConfiguration _config;

        public TokenController(IConfiguration config)
        {
            _config = config;
        }

        [AllowAnonymous]
        [HttpPost]
        public IActionResult CreateToken([FromBody] LoginModel login)
        {
            string? id = login.Username;
            string? pw = login.Password;
            IActionResult response = Unauthorized();

            if (!string.IsNullOrEmpty(id) && !string.IsNullOrEmpty(pw))
            {
                if (VerifyUser(id, pw))
                {
                    var tokenString = BuildToken();
                    response = Ok(new { token = tokenString });
                }
            }

            return response;
        }

        private bool VerifyUser(string id, string pw)
        {
            // verify id and password sent from user (code omitted)            
            // simply return true in this sample
            return true;
        }

        private string BuildToken()
        {
            var key = new SymmetricSecurityKey(
                Encoding.UTF8.GetBytes(_config["Jwt:Key"]!));

            var creds = new SigningCredentials(
                key, SecurityAlgorithms.HmacSha256);

            var token = new JwtSecurityToken(
                issuer: _config["Jwt:Issuer"],
                audience: _config["Jwt:Issuer"],
                claims: null,
                notBefore: null,
                expires: DateTime.Now.AddMinutes(30),
                signingCredentials: creds);

            return new JwtSecurityTokenHandler().WriteToken(token);
        }

        public class LoginModel
        {
            public string? Username { get; set; }
            public string? Password { get; set; }
        }
    }
}