How to add relationships from the IdentityUser class in ASP.NET Core 3.1 Identity? (Code First)

Question

I have a Asp.Net Core MVC 3.1 project. I want to join appointment and patient tables with code first technique however the foreign key column is null.

My Patient Class;

public class Patient : IdentityUser
{
     [NotMapped]
     public List<Appointment> Appointments { get; set; }
}

And my Appointment class, I have one to many relationship with patient and appointment.

public class Appointment
{
    [Key]
    public int ID { get; set; }

    public string PatientID { get; set; }
    public Patient Patient { get; set; }
}

If I add "required" annotations to the foreign key not saving appointment.

public Result<AppointmentVM> CreateAppointment(AppointmentVM model)
    {
        if (model != null)
        {
            try
            {
                var appointment = _mapper.Map<AppointmentVM, Appointment>(model);
                _unitOfWork.appointmentRepository.Add(appointment);
                _unitOfWork.Save();
                return new Result<AppointmentVM>(true, "Successful");
            }
            catch (Exception ex)
            {
                return new Result<AppointmentVM>(false, "Error" + ex.Message.ToString());
            }
        }
        else
            return new Result<AppointmentVM>(false, "Not null");
    }

How can i solve this problem?

public class AppointmentVM
{
    public int AppointmentID { get; set; }

    public string PatientID { get; set; }
    public PatientVM Patient { get; set; }
}


public class AppointmentRepository : Repository<Appointment>, IAppointmentRepository
{
    private readonly ClinicContext _ctx;

    public AppointmentRepository(ClinicContext ctx) : base(ctx)
    {
        _ctx = ctx;
    }
}