Chapter 1 What’s New for FortiOS 5.2.1 : Security Profiles : Web Filtering : HTTPS for Warnings and Authentication
  
HTTPS for Warnings and Authentication
HTTPS protocol can now be used when sending web filtering warnings or requiring a user to authenticate, including authentication for web filter overrides.
Syntax
config webfilter fortiguard
set ovrd-auth-https enable
set warn-auth-https enable
end