When using the the GoToWebinar plugin, it will generate a select list based on synced contact history. Webinars that have not started syncing contacts to Campaign Studio will not show up in the list.
There are two possible workarounds here:
- Manually upload lists of registrants/attendees into segments and use those conditions
- Register a "fake" contact to the webinar and let it sync to Campaign Studio. That should create a sync record which will then show up in the dropdown list.