Thank you! This actually helped me find it and insert the needed key.
But it's not all ready to go yet. When trying to send out email from Current User I now get "Resource could not be discovered." error.
This could not be related to permissions since I'm admin with SUPERuser permissions.
Or does this feature need 365 Online Exchange/Hybrid deployment and therefore is looking resource at Exchange online and cant find it? I can't find information on it.