how to create a new user when inheriting from IdentityUser

I have the following class derived from IdentityUser. Person class is stored in AspNetUsers table in the database, and everything looks good on the database-side.

public class Person : IdentityUser
{
    [Required]
    [StringLength(50, ErrorMessage = "First name cannot be longer than 50 characters.")]
    public string FirstName { get; set; }

    [Required]
    [StringLength(150, ErrorMessage = "Last name cannot be longer than 150 characters.")]
    public string LastName { get; set; }
}

My questions is about creating a new user (Person) that has firstname and lastname. I guess, first I need to do this:

var user = new IdentityUser() { UserName = "testing" };
IdentityResult result = userManager.Create(user,"123456");

This will insert a new row to AspNetUsers table with null Firstname and Lastname fields. Then, by using LinQ, I need to update Firstname and Lastname fields of the existing record. Is this approach reasonable? Is there any other recommended way of doing this?