1 - Create a public folder mailbox.
2 - Create a public folder.
3 - Mail-enable the public folder.
Reference:
https://docs.microsoft.com/en-us/exchange/collaboration-exo/public-folders/create-public-folder
https://docs.microsoft.com/en-us/exchange/collaboration-exo/public-folders/enable-or-disable-mail-for-public-folder