How to associate a Volunteer Opportunity with a Volunteer Event
While you could create a volunteer opportunity for an event from its related list, this is the hard way. Instead use the volunteer opportunity wizard to create a volunteer opportunity that will take place during your event, and THEN associate it with the event.
See Working with Volunteer Opportunities for full instructions on creating volunteer opportunities.
Associate your opportunity to the event in the Special Event Information section of the opportunity record.
Edit the Volunteer Opportunity field "Volunteer Event" and use the lookup to select one of your existing volunteer events. Once you select the appropriate event -- the opportunity will become part of that event and be linked to it on the public site. **Please note that the dates for any volunteer opportunities that you associate with a volunteer event should fall within the start date and end date specified for the volunteer event.
The field "Volunteer Event URL" will be automatically filled in for you when you save the record. This will give you a link directly to the event page on the public site.