The Users Without Passwords Project (UWPP) persists the ChallengeGuid property on the server for response verification with a TempData attribute. TempData is a storage container for data that needs to be available to a separate HTTP request. Once the value has been read, it is marked for deletion and is therefore not available for subsequent requests. See TempData In Razor Pages. This demo transfers the LoginName property from Page 1 to Page 2. Page 2 persists the LoginName and ChallengeGuid properties on the server between the initial request and the first callback with TempData attributes. The TempData attributes do not persist after the first callback. See ASP.NET Core 5.0 - The TempData Challenge.

TempDataDemoP1.cshtml.cs

public class TempDataDemoP1Model : PageModel
{
    [BindProperty]
    [Required]
    [Display(Name = "Login Name")]
    [StringLength(32, ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 8)]
    public string LoginNameInput { get; set; } = string.Empty;

    public void OnGet()
    {
        TempData.Clear();
    }

    public IActionResult OnPost()
    {
        if (!ModelState.IsValid) return Page();

        TempData["LoginName"] = LoginName;

        return RedirectToPage("./TempDataDemoP2");
    }
}