Step 1
Go to Outlook Manage Add-ins:
https://outlook.office.com/?path=/options/manageapps
(you'll have to sign in with your Office 365 account)
Step 2
Select My add-ins
Scroll to Custom add-ins and choose Add From URL...
Step 3
Enter URL
https://freebusy.io/outlook/alternate.xml
Click Ok, accept the warning and then click Install
Step 4 - Outlook desktop
When you compose or reply to an email in Outlook desktop and when you schedule or reschedule a meeting in Outlook desktop, you'll be able to open the FreeBusy add-in from the ribbon. If you see two FreeBusy icons, open the one labeled "FreeBusyAlt".
Step 4 - Outlook web
When you compose or reply to an email in Outlook web you'll be able to open the FreeBusy add-in from the lower toolbar (might be necessary to search it in the drop-down menu). If you see two FreeBusy icons, open the one labeled "FreeBusyAlt".
When you schedule or reschedule a meeting in Outlook web, you'll be able to open the FreeBusy add-in from the upper toolbar in the event editing extended dialog:
