How to set a unique constraint using Entity Framework Code First?

I'm attempting the following:

To prevent someone from applying to the same posting more than once, add the UNIQUE constraint IX Unique Applicatation as a business rule to the Application Class.

I'm attempting to use this to

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Web;

namespace solution_MVC_HR.Models
{
    public class Application
    {
        public int ID { get; set; }

        [Display(Name = "Covering Comments")]
        [Required(ErrorMessage = "You cannot leave the summary comments blank.")]
        [StringLength(2000,ErrorMessage ="Summary must be between 20 and 2000 characters",MinimumLength =20)]
        [DataType(DataType.MultilineText)]
        public string Comments { get; set; }

        [Required(ErrorMessage ="You must specify the job posting applied for.")]
        public int PostingID { get; set; }

        public virtual Posting Posting { get; set; }

        [Required(ErrorMessage = "You must specify the applicant applying to the job posting.")]
        public int ApplicantID { get; set; }

        [Index(IsUnique = true)]
        public virtual Applicant Applicant { get; set; }

    }
}

I receive the following when I add-migrate IX Unique Applicatation:

public partial class IX_Unique_Applicatation : DbMigration
    {
        public override void Up()
        {
        }

        public override void Down()
        {
        }
    }

It is entirely empty (obviously, there should be something in the void Up and void Down). I've experimented with it in a few other sectors. I don't know where I'm going wrong, so any assistance would be highly appreciated.

I've tried using this kind of code, however it keeps giving me an issue regarding my Unique on the ID column.

[Index("IX_MyTwoColumns", 1, IsUnique = true)]
public int UserAttributeId { get; set; }
[ForeignKey("UserAttributeId")]
public virtual UserAttribute UserAttribute { get; set; }