jerguero-cs/Pages/New.cshtml.cs

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;

namespace Jerguero.Pages
{
    public class NewDefinitionModel : PageModel
    {
        public IActionResult OnGet()
        {
            if (User.Identity?.IsAuthenticated != true)
            {
                return RedirectToPage($"/Login", new {redirect = "/New"});
            }

            return Page();
        }
    }
}