page.cs

public async Task<IActionResult> OnPost(CatalogItemViewModel productDetails)
{
    if (productDetails?.Id == null)
    {
        return RedirectToPage("/Index");
    }

    await SetBasketModelAsync();

    var result = await _basketService.AddItemToBasket(BasketModel.Id, productDetails.Id, productDetails.Price, productDetails.Quantity);

    await SetBasketModelAsync();

    return result.Match<IActionResult>(
        quantity => RedirectToPage(),
        validationError => new BadRequestResult());
}