Automatic crossposting is a Facebook feature that will automatically present your broadcast on more than one Business Page, even business pages you don't administer.
Note that this does not apply to Groups, and that comments and reactions will not show up between the cross-posted Business Pages.
Setting up Automatic Cross-Posting on the Facebook Website
Business pages that have crossposting relationships will be listed automatically. Establish cross-posting relationships using the Page’s Crossposting settings on the Facebook website. To find the Crossposting setting:
Switch to a Page profile that you administer on facebook.com
Pick Settings & privacy from the top-right menu
In the left-hand sidebar, click Videos
Then you'll see a Crossposting section