How to declare foreign key in Entity Framework

There are 2 ways to declare foreign key in entity framework. Assuming we have 2 entities aspnet_Users and Person, which are link via UserId from aspnet_Users. 1.)

class Person {
  public Guid UserId { get;set; }
  [ForeignKey("UserId")]
  public virtual AspnetUser AspnetUser { get; set; }
}

2.)

  [ForeignKey("AspnetUser")]
  public Guid UserId { get;set; }  
  public virtual AspnetUser AspnetUser { get; set; }