I'm finishing up an add-on that integrates with a third-party service. It requires that you authorize my add-on to work with your account (or possibly your client does to work with theirs).
I think it makes most sense to do this through the control panel since only admins get access to it. Also, this way the authorization flow doesn't require any one-time use templates that should be password-protected. Where would you expect to see the link to authorize the add-on?