How to updating referance table on record with same userid with customerid

See more:

MVC5

entity-framework

C#7.0

I have a reference table that when a user is created it automatically, by Trigger, puts a record with the userId. When a Customer adds Company info I would like to update that record with CustomerId. Below where (private readonly int CustomerId) is this has a message that reads:
Field 'CustomerController.CustomerId' is never assigned to, and will always have its default value 0

So perhaps it is updating the record with 0. I cannot tell because on record creation with the trigger I put a zero in the columns as I have them set to not null.
Thank you for your help.

What I have tried:

Controller:

private readonly int CustomerId;
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult CreateProfile([Bind(Include = "CustomerId,CustomerName")] CustomerNames customerNames)
        {
            if (ModelState.IsValid)
            {
                db.CustomerNames.Add(customerNames);
                var user = User.Identity.GetUserId();
                var companyId = (from p in db.UserJoin where p.UserId == user select p).First();
                companyId.CustomerId = CustomerId;
                db.SaveChanges();
                return RedirectToAction("AddBillingAddress", "BillingAddresses");
            }

            return View(customerNames);
        }

Model:

public class UsersToAddresses
{
    [Key]
    public string UserId { get; set; }
    public int BillToId { get; set; }
    public int ShipToId { get; set; }
    public int CustomerId { get; set; }
}