HomeController.cs

public async Task<IActionResult> Index()
{
    var principal = User.Identity as ClaimsIdentity;
    var idClaim = principal.Claims.Where(i => i.Type == "http://marcusclasson.com/claims/id")
        .SingleOrDefault();

    if(idClaim == null)
    {
        principal.AddClaim(new Claim("http://marcusclasson.com/claims/id", "MyCustomId"));
        await HttpContext.SignOutAsync(CookieAuthenticationDefaults.AuthenticationScheme);
        await HttpContext.SignInAsync(User);
    }
    return View();
}