If you manually add a subscriber, the assumption is that you already have permission from them to add them (i.e. importing a list from elsewhere) and incentives won't be sent, because the incentive email is where they give that permission to opt-in. Since you've already added them to your mailing list manually, I'd suggest you just write this person an email from the email address you use to send broadcasts, and attach the freebie/incentive.

Emily MillsCommunity Guru
Illustrator, Author, Course Instructor