Lightning: Attach Calendar Invite (ICS) To Email
Click the gear icon
Click Setup
Search "Custom Settings"
Click Custom Settings
Click "Manage" for Calendar Config
Click Edit
Check the "Attach Calendar Invite To Email" checkbox
Click Save
Click the gear icon
Click Setup
Search "Custom Settings"
Click Custom Settings
Click "Manage" for Calendar Config
Click Edit
Check the "Attach Calendar Invite To Email" checkbox
Click Save