According to an update posted on the YouTube Help site, users will no longer be able to schedule Hangouts On Air events via Google+ accounts in the coming weeks. Starting September 12, Hangouts On Air events will be scheduled using YouTube Live.
Hangouts On Air will move from Google+ to YouTube Live on September 12. If you want to schedule new Hangouts On Air you will need to use YouTube Live. Events cannot be scheduled on Google+ after September 12 and you will need to move existing events scheduled to happen after September 12 to YouTube Live.
The post includes instructions for setting up a Hangouts On Air events with YouTube Live, directing users to the Live Streaming Events in Creator Studio.
Google confirmed users will be able to access recorded events on YouTube, while past event content will be available via Google+ in a read-only format in the Activity Log. Also, any events currently scheduled after September 12 will need to be moved to YouTube Live.