Yeah that is strange. I am not sure what it would be. You created your signature key from the same place you created your transaction key, correct? You answered the question with Simon? I am sure you did because you would be getting back a null value rather than an empty string. You would have a transaction key, login, signature key, and (if you use accept.js) a public client key. So 4 api credentials total. A consolation prize if you can’t figure this out is that the modern integration methods don’t get as much benefit from this as the old ones. You can do without it, I think.