How to use User.Identity.GetUserId() in Web Api 2

Question

I am using a post method where comedyId is coming from body of the request, while using postman i am getting redirected to login page again and again even after login.

Why login is not working for Api and User.Identity.GetUserId() is also returning null value ?

 namespace ComedyCentral.Controllers.Api
    {
        [Authorize]
        public class AttendancesController : ApiController
        {
            private ApplicationDbContext _context;

            public AttendancesController()
            {
                _context = new ApplicationDbContext();
            }

            [HttpPost]
            public IHttpActionResult Attend([FromBody] int comedyId)
            {
                var userId = User.Identity.GetUserId();


                if (_context.Attendances.Any(a => a.AttendeeId == userId && a.ComedyId == comedyId))
                    return BadRequest("The attendance already exists.");

                var attendance = new Attendance
                {
                    ComedyId = comedyId,
                    AttendeeId = userId
                };
                _context.Attendances.Add(attendance);
                _context.SaveChanges();

                return Ok();
            }
        }
    }