I’ve written a userscript that uses the web service and so needs a key. The link to register an application on Web Service | AcoustID points at flask:apps.new_application which doesn’t lead to any page and so doesn’t work.
For now I “collected” a bunch of keys from the internet and rotate through them (so that hopefully doesn’t impact the keys’ owners), but if I ever wanted to publish the script it probably should have its own key.