When attendees register for your Eventbrite event you will probably want them to have the option to opt in to future communications from you. This article from Eventbrite shows you how to gather those opt-ins using questions on the Eventbrite order.
If you’d like the opt-in to be recorded against the matching constituent in Donorfy, this may be set up when an Eventbrite event is linked to a Donorfy campaign. The Eventbrite tab for a Campaign allows up to 3 questions from the Eventbrite order to be mapped to a Channel and/or a Purpose in Donorfy.
Note that only questions having recognisable Yes/No type answers will be presented for mapping in Donorfy.