Setup one to many relationship in EF6

// You can add profile data for the user by adding more properties to your ApplicationUser class, please visit http://go.microsoft.com/fwlink/?LinkID=317594 to learn more.
    public class AppUser : IdentityUser
    {
        public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<AppUser> 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;
        }

        public int? DomainId { get; set; }

        [ForeignKey("DomainId")]
        public virtual Domain Domain { get; set; }
    }

public class Domain
    {
        [Key]
        public int DomainId { get; set; }

        [Required]
        [MaxLength(100)]
        public string Name { get; set; }

        public virtual ICollection<AppUser> AppUsers { get; set; }
    }