How to offer users sign in during purchase?

I am working on cinema website. We allow users buy tickets without registration, but the main business requirement is motivate users sign in/sign up during purchase process - after click on seat you see notification, that offer you to sign in, sign up or continue as guest.

So I am confused at which step to offer users sign in and how to do that with minimal risk of breaking the deal. Should it be pop-up or just text notification? Should it be when user choose screening time, choose seats, or make checkout?