Registration will be done using a webform which can be accessed from this page:
https://disneyvacationclub.disney.go.com/discounts-perks-offers/member-events/moonlight-magic-epcot/
You might need to log on to see the registration button once the booking window opens.
You may (or may not) receive an email from DVC with a direct link to registration page.
The form will ask for basic information like:
Are you staying on property. If yes, which resort.
Member Name
Membership Number
Which Event Date (if multiple)
Number of Guests (including member)