Modified from https://code.msdn.microsoft.com/How-to-authorization-914d126b

TokenAuthController.cs

using System;
using System.IdentityModel.Tokens.Jwt;
using System.Net;
using System.Net.Http;
using System.Security.Claims;
using System.Security.Principal;
using System.Text;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.IdentityModel.Tokens;
using Newtonsoft.Json;
using Snaggy.Auth;
using Snaggy.Models;
using Snaggy.Models.TokenAuthViewModels;

namespace Snaggy.Controllers
{
    [Route("api/[controller]")]
    [Produces("application/json")]
    public class TokenAuthController
    {
        private readonly UserManager<User> _userManager;
        private readonly SignInManager<User> _signInManager;
        private readonly IConfiguration _configuration;

        /// <summary>
        /// Constructs a new instance of <see cref="TokenAuthController"/>
        /// </summary>
        /// <param name="userManager"></param>
        /// <param name="signInManager"></param>
        /// <param name="configuration"></param>
        public TokenAuthController(UserManager<User> userManager, SignInManager<User> signInManager, IConfiguration configuration)
        {
            _userManager = userManager;
            _signInManager = signInManager;
            _configuration = configuration;
        }

        /// <summary>
        /// Provides for generation of a JWT bearer token for a given user/pass
        /// </summary>
        /// <param name="tokenRequest">The information required to generate a token</param>
        /// <response code="200">Token details, or null</response>
        [HttpPost]
        public async Task<TokenResult> GetAuthToken([FromBody]LoginModel tokenRequest)
        {

            var user = await _userManager.FindByNameAsync(tokenRequest.Username);
            var result = await _signInManager.CheckPasswordSignInAsync(user, tokenRequest.Password, lockoutOnFailure: false);

            if (result.Succeeded)
            {
                var now = DateTime.Now;
                var expires = now + TimeSpan.FromDays(5);

                var principal = await _signInManager.CreateUserPrincipalAsync(user);
                var identity = new ClaimsIdentity(principal.Identity, principal.Claims);
                return new TokenResult
                {
                    Expires = expires,
                    Requested = now,
                    Token = GenerateToken(identity, expires),
                    TokenType = "Bearer"
                };
            }

            // Ideally, we'd use content negotiation here, and expose a problem detail object.
            return null;
        }

        private string GenerateToken(ClaimsIdentity identity, DateTime expires)
        {
            var handler = new JwtSecurityTokenHandler();
            var tokenDetails = new TokenDetails(_configuration);
            var securityToken = handler.CreateToken(new SecurityTokenDescriptor
            {
                Issuer = tokenDetails.Issuer,
                Audience = tokenDetails.Audience,
                SigningCredentials = tokenDetails.SigningCredentials,
                Subject = identity,
                Expires = expires
            });
            return handler.WriteToken(securityToken);
        }

    }
}

TokenDetails.cs

using System.Text;
using Microsoft.Extensions.Configuration;
using Microsoft.IdentityModel.Tokens;

namespace Snaggy.Auth
{
    internal class TokenDetails
    {
        internal string Path { get; set; }
        internal string Issuer { get; set; }
        internal string Audience { get; set; }
        internal SymmetricSecurityKey SigningKey { get; set; }
        internal SigningCredentials SigningCredentials { get; set; }

        internal TokenDetails(IConfiguration configuration)
        {
            Path = configuration.GetSection("jwt:path").Value;
            Audience = configuration.GetSection("jwt:audience").Value;
            Issuer = configuration.GetSection("jwt:issuer").Value;

            SigningKey = new SymmetricSecurityKey(Encoding.ASCII.GetBytes(configuration.GetSection("jwt:secret").Value));
            SigningCredentials = new SigningCredentials(SigningKey, SecurityAlgorithms.HmacSha256);
        }
    }
}

TokenResult.cs

using System;

namespace Snaggy.Models.TokenAuthViewModels
{
    public class TokenResult
    {
        public DateTime Requested { get; set; }
        public DateTime Expires { get; set; }
        public string TokenType { get; set; }
        public string Token { get; set; }
    }
}