To toggle the broadcast option, click the screen icon in the feed form. It will be available as soon as your message has any content.
If you enable broadcasting, you will be presented with a popup as soon as you click Post:
The popup willl give you two options: broadcasting to all- or a selection of the available screens.
Screens available to you are limited based on team access:
Your company uses team access and you are a company admin: All screens in the teams you have team access to
Your company uses team access and you are a regular user: All screens set up for your team
Your company doesn't use team access: All screens in the company
Post the message, and it will show on all the screens chosen.