Siri currently only works with certain native iOS services.
But Evernote explains over at its blog how it can integrate with Siri despite being a third party service.
All you need to do is add a contact named “Evernote” with your incoming Evernote email address.
Activate Siri and say. “Send Evernote an email.”
Then speak the content of the note you want to create and send the email. It’s instantly brought into your Evernote account.