Secure https: access is a standard feature which was added two years ago. See:
https://fastmail.blog/2018/09/06/sec...bsite-support/
When you create (or edit) a website in the
Files>Websites area, you are given this choice:
HTTPS
Securely encrypt all communication with this site
This is currently only available for websites at custom domains.
As noted above, the only way this will work is if you own your own (custom) domain. Fastmail then automatically handles the security certificate for you. I have been using this feature since it was introduced.
Bill