Why there is no redirect from HTTP to HTTPS on access to the Plesk via customized URL (i.e.
Note: The domain example.com does not exist but set in Plesk > Tools & Settings > Customize Plesk URL.
This is default Plesk behavior. There is no redirect from HTTP to HTTPS for the domain specified in Customize Plesk URL.
Vote the following feature request from our Official Plesk UserVoice channel as top-voted suggestions are likely to be included in future Plesk versions.
As a workaround:
Create the corresponding subscription in Plesk:
Create a new subscription in Subscriptions > Add Subscription with the name example.com
Go to Subscriptions > example.com > Hosting settings
Tick the options SSL/TLS support and Permanent SEO-safe 301 redirect from HTTP to HTTPS
Click the OK button