If you do not have an Office 365 account but have the desktop app, depending on the version you may be able to add the extension through the add-ins store.
1. Go to the Store on the home tab
2. Search for Highfive, then click on Add:
3. Once it's added, click on the highfive icon that pops up on the left side while in a new event:
4. Click on Sign In, in the sidebar that pops up.
5. When the browser pops up, click on Allow.
You will then be good to go with setting up the new and improved scheduling extension for Highfive. You will see the following pop up in the sidebar when you click on the Highfive Icon:
Having trouble with the Highfive extension? Reach out to the Highfive Support team at firstname.lastname@example.org for assistance.