I realize now that I tried this before. On Kiwi I can't find the nos2x options page to set up my keys.
Discussion
set up private key: Extensions (kiwi://extensions) -> nos2x -> Details -> Extension options -> private key -> save
When you get it working a couple of tips: sometimes you authorize something and seems that nothing happened, it did, just close the permissions tab. Some other times the webapp you are trying to use seems to not ask for permission, it did, switch to the right tab.