How to create Foreign key in EF Code First for AspNetUser Table created by Identity

I have the following code: 
public class ApplicationUser : IdentityUser     {         [StringLength(30, ErrorMessage="First Name should not exceed 30 characters")]         [Required(ErrorMessage = "Please enter your First Name")]         public string FirstName { get; set; }         [StringLength(30, ErrorMessage = "Last Name should not exceed 30 characters")]         [R ...