Get current logged in user id in ASP.Net Core

Hi I created project using asp.net core 2.2 razor page and i try to add login user id to some tables my codes :

Model :

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Threading.Tasks;

namespace LibraryManage.Models
{
    public class TestUserId
    {
        public int Id { get; set; }
        [Required]
        public string Name { get; set; }
        [DataType(DataType.Date)]
        public DateTime DateOfBearth { get; set; }

        [ScaffoldColumn(false)]
        public string UserId { get; set; }
        [ForeignKey("UserId")]
        public virtual ApplicationUser ApplicationUser { get; set; }
    }
}

Create.cshtml :

@page
@model LibraryManage.Pages.TestUserIds.CreateModel

@{
    ViewData["Title"] = "Create";
    Layout = "~/Pages/Shared/_Layout.cshtml";
}

<h1>Create</h1>

<h4>TestUserId</h4>
<hr />
<div class="row">
    <div class="col-md-4">
        <form method="post">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <div class="form-group">
                <label asp-for="TestUserId.Name" class="control-label"></label>
                <input asp-for="TestUserId.Name" class="form-control" />
                <span asp-validation-for="TestUserId.Name" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="TestUserId.DateOfBearth" class="control-label"></label>
                <input asp-for="TestUserId.DateOfBearth" class="form-control" />
                <span asp-validation-for="TestUserId.DateOfBearth" class="text-danger"></span>
            </div>
          
            <div class="form-group">
                <input type="submit" value="Create" class="btn btn-primary" />
            </div>
        </form>
    </div>
</div>

<div>
    <a asp-page="Index">Back to List</a>
</div>

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

Create.cshtml.cs :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.AspNetCore.Mvc.Rendering;
using LibraryManage.Data;
using LibraryManage.Models;
using System.Security.Claims;
using Microsoft.AspNetCore.Authorization;

namespace LibraryManage.Pages.TestUserIds
{
    [Authorize]
    public class CreateModel : PageModel
    {
        private readonly ApplicationDbContext _context;

        public CreateModel(ApplicationDbContext context)
        {
            _context = context;
        }

        public IActionResult OnGet(string userId)
        {
           
            var claimsIdentity = (ClaimsIdentity)User.Identity;
            var claim = claimsIdentity.FindFirst(ClaimTypes.NameIdentifier);
            userId = claim.Value;
            TestUserId.UserId = userId;
          
            return Page();
        }

        [BindProperty]
        public TestUserId TestUserId { get; set; }

        public async Task<IActionResult> OnPostAsync()
        {
            if (!ModelState.IsValid)
            {
                return Page();
            }

            _context.TestUserId.Add(TestUserId);
            await _context.SaveChangesAsync();

            return RedirectToPage("./Index");
        }
    }
}

But not work It always display userid as null value