As you may already know, Teams is going to replace Skype for Business (in a way or another) on Office 365.
As part of the ongoing improvement of Teams, a new capability is now in preview called: Live Events.
The idea of Teams Live Event is to setup and host Teams meeting with large audience; sound familiar? Yep, it used to be (or still be for some time anyway) Skype for Business Broadcast.
Teams Live Events is available in North America (US, Canada, Mexico), APAC and EMEA in preview and you need to be licensed for Skype for Business Online (apparently Teams license is not sufficient).
You also need to have turn on the ability to create Private meeting (from the Office 365 Admin portal\Settings\Services & Add-ins\Teams)
Schedule the Live Event Meeting
Using your Teams client go to the Meetings section and Schedule a meeting and open the contextual menu to select New live event (preview)
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Then you can fill your event details as for any event
You will also note you are going to be the producer by default (but you can switch), and can you also grant this permission to someone else
DO NOT INVITE yet your external participants; when you will hit the Next button you will be able to define who will be able to watch the event
As you may see, the public option is not available; this is because no policy has been setup to allow it (see below).
Live Events Policy
As stated above, you may need to setup a Live Events policy to at least allow public access or to enable/disable recording or transcription.
To setup a Live Events policy you need to access the new Teams & Skype for Business administration portal; this is one can be a little bit tricky 
To get the new portal, you may have to go to your Office 365 Admin portal\Settings\Services & Add-ins\Teams and take a look at the notification (or you can also use the direct URL https://admin.teams.microsoft.com)
Then from this new administration portal (not all the settings are available yet), you need to open the Meetings\Live event policies
You may have a furtive error message being displayed
Error We couldn’t get any Live event policies
because you have not yet setup a policy
Then you just need to click on New policy to setup your first Live Events policy
To get back the public audience not enabled, you need to select Attendee visibility mode to Everyone
As said, this is a preview so you may face some errors.