How to Create Foreign key ??

Question

I am using code first approach to create database.
Then  i am try to Update Database in Package console Manger using
Update-Database -Verbose  command error show :-
The navigation property 'UserId' is not a declared property on type 'User'. Verify that it has not been explicitly excluded from the model and that it is a valid navigation property.

[Table("UserProfile")]

    public class UserProfile

    {

        [Key]

        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]

        public int UserId { get; set; }

        public string UserName { get; set; }

          public virtual ICollection<User> Usera { get; set; }

 

    }

  public class User

    {

        [Key]

        public long u_ID { get; set; }

        //public int UserId { get; set; }

        [Required(ErrorMessage = "Please Enter Headline name ")]

        [Display(Name = "Headline Profile")]

        public string head_profile { get; set; }

        [Required(ErrorMessage = "Please Enter Full Name")]

        [Display(Name = "Full Name:")]

        public string full_name { get; set; }

   
       [ForeignKey("UserId")]

        public int UserId { get; set; }

        public virtual UserProfile UserProfilefk { get; set; }

    }


How to Create Foreign key