Add custom index to entity that inherits from User, which inherits from IdentityUser in EF

I'm making ASP.NET MVC Project with user identity. My User class now inherits from IdentityUser:

public class User : IdentityUser
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Patronymic { get; set; } // Отчество
    public string Avatar { get; set; }
    public DateTime? DateOfBirth { get; set; } // Дата рождения
    public DateTime? RegisterDate { get; set; } // Actualy it is a dateTime :)
    public bool? Sex { get; set; }

    public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<User> manager)
        // Note the authenticationType must match the one defined in CookieAuthenticationOptions.AuthenticationType
        var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
        // Add custom user claims here
        return userIdentity;

Then I made the Doctor class, which inherits from it:

public class Doctor : User
    public int DoctorId { get; set; }
    public string CurriculumVitae { get; set; }

As you can see, I have added the custom index field DoctorId. But UserManager can't create the entities So the question is HOW CAN I ADD THE CUSTOM INT ID INDEX?