Need help with "creating an authentication cookie"

public async Task<IActionResult> OnPostAsync(string returnUrl = null)
{
    ReturnUrl = returnUrl;

    if (ModelState.IsValid)
    {
        // Use Input.Email and Input.Password to authenticate the user
        // with your custom authentication logic.
        //
        // For demonstration purposes, the sample validates the user
        // on the email address [email protected] with 
        // any password that passes model validation.

        var user = await AuthenticateUser(Input.Email, Input.Password);

        if (user == null)
        {
            ModelState.AddModelError(string.Empty, "Invalid login attempt.");
            return Page();
        }
        
         ... too much code, I can't add it
        };

        await HttpContext.SignInAsync(
            CookieAuthenticationDefaults.AuthenticationScheme, 
            new ClaimsPrincipal(claimsIdentity), 
            authProperties);

        _logger.LogInformation("User {Email} logged in at {Time}.", 
            user.Email, DateTime.UtcNow);

        return LocalRedirect(Url.GetLocalUrl(returnUrl));
    }

    // Something failed. Redisplay the form.
    return Page();
}