How do you set the "Auditorium" mode for a Skype Meeting Broadcast?

I don't think there is an actual setting, but there are ways that you can setup a Skype meeting to make it more conducive to a presentation than a collaborative session.   

When you create the meeting, under meeting options, you can specify who joins as a presenter vs. a attendee.   Presenters can share content and control who's in the meeting.

Once the meeting starts, if you are a presentor, you can use the Participant actions to Mute the Auduience (all attendees) and control some other aspects of the experience.

There is also the Skype Meeting Broadcast which is a differene service designed for really large meetings:  https://support.office.com/en-us/article/What-is-a-Skype-Meeting-Broadcast-c472c76b-21f1-4e4b-ab58-329a6c33757d