When you sign up to use Adobe's email service you'll get an email with the number. If you've already done that and can't find it, check your spam filter.
Just to elaborate, as you have added the email id after selecting Adobe email services. You will get an email from 'firstname.lastname@example.org'. In this mail there will be an alphanumeric code. This is the verification code asked by Organizer. If this email is not in the inbox then check your spam folder.