Click on the Internet Certificate Download button to download the certificate to your device.
When the download is complete go to your Chromebook settings
Scroll down and click on Show advanced settings
Click on Manage Certificates
Click on the Authorities tab
Click on Import
Browse to your Downloads
Click on the drop down menu next to the open button to select All Files
Select the downloaded Fortinet Internet certificate
Click Open & trust all 3 settings and then click OK
Click Done to finish the install of the certificate
You are now able to browse to secure sites such as Office 365