@Anssi the part after ?title= is volume's url-friendly name that you can copypaste from volume's page. Alternatively, you can get the full link here or elsewhere if someone posted it.
Either way, you'll get "Redeem a credit for ?" message because that page won't render properly unless you navigated there from volume's page. It even looks that way if you're member and middle-clicked the purchase button. There are server-side checks (whether a book with that name exists, you don't have a copy and have enough credits) when you click "Redeem" button so nothing too bad should happen even if you misspell but I can't promise anything.
edit: also, you can currently get a free trial to gain The Button privilege.