Set up for public folders in Outlook Online
Important prerequisite: The following steps should be executed inside the Azure AD application which is created according to the instructions from the Set Up of the Azure AD Application
Request API Permissions
Select APIs my organization uses
Select Office 365 Exchange Online as shown in the screenshot

Choose Application permissions, then full_access_as_app

Add permissions, then Grant admin consent!
Create a service account for amberSearch with Mailbox and grant read permissions for all relevant public folders to this account.