Related Questions:
Why isn't my vent showing up on my contract?
How can I add an Event to an existing BEO?
Can I link multiple Events to an existing BEO?
Procedure:
Within the existing Document it will show you all the Events that can be linked or unlinked next to the Linked Events header. You can link one or multiple Events.
Open the Document
Select the dropdown menu next to Linked Events near the top of the Document
Click the Checkboxes next to the Event or Events you wish to link
If desired, make one of those Events the Primary Event