asp.net identity claim fullname in view page

I am new at asp.net Identity 2.0 and I want to show my FullName in Razor view page instade of Username.

So I added new property to IdentityUser.

public class ApplicationUser : IdentityUser
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string FullName => $"{FirstName} {LastName}";
}

Default AccountController contains a Login Method:

    [HttpPost]
    [AllowAnonymous]
    [ValidateAntiForgeryToken]
    public async Task<ActionResult> Login(LoginViewModel model, string returnUrl)
    {           
        var result = await SignInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, shouldLockout: false);

        switch (result)
        {
            case SignInStatus.Success:
                return RedirectToLocal(returnUrl);
            case SignInStatus.LockedOut:
                return View("Lockout");
            case SignInStatus.RequiresVerification:
                return RedirectToAction("SendCode", new { ReturnUrl = returnUrl, RememberMe = model.RememberMe });
            case SignInStatus.Failure:
            default:
                ModelState.AddModelError("", "Invalid login attempt.");
                return View(model);
        }
    }

I edited this login method

    public async Task<ActionResult> Login(LoginViewModel model, string returnUrl)
    {           
        var result = await SignInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, shouldLockout: false);

        //-------------------------------------------------------
        if (result == SignInStatus.Success)
        {
            var user = await UserManager.FindByEmailAsync(model.Email);
            if (user != null)
            {
                await UserManager.AddClaimAsync(user.Id, new Claim("FullName", user.FullName));
            }
        }
        //--------------------------------------------------------

        switch (result)
        {
            case SignInStatus.Success:
                return RedirectToLocal(returnUrl);
            case SignInStatus.LockedOut:
                return View("Lockout");
            case SignInStatus.RequiresVerification:
                return RedirectToAction("SendCode", new { ReturnUrl = returnUrl, RememberMe = model.RememberMe });
            case SignInStatus.Failure:
            default:
                ModelState.AddModelError("", "Invalid login attempt.");
                return View(model);
        }
    }

And I created an extention method to read FullName from Razor view:

public static class IdentityExtensions
{
    public static string GetFullName(this IIdentity identity)
    {
        var claim = ((ClaimsIdentity) identity);

        return claim.FindFirst("FullName");            
    }
}

But FullName allways coming Null

<ul class="nav navbar-nav navbar-right">
    <li>
        @Html.ActionLink("Hello " + User.Identity.GetFullName() + "!", "Index", "Manage", routeValues: null, htmlAttributes: new { title = "Manage" })
    </li>
    <li><a href="javascript:document.getElementById('logoutForm').submit()">Log off</a></li>
</ul>