Help:Extension:SecureLinkFixer
The SecureLinkFixer extension automatically rewrites some URLs to use HTTPS.
Which domains?
[edit]The domain list comes from Mozilla's HSTS preload list. Websites can submit themselves to be added. You can see the list of domains in Gerrit (warning: large page).
How to add a new domain?
[edit]Website operators can submit themselves to be added to the HSTS preload list.