A domain that exist in Plesk (e.g. example.com) and is secured with an SSL certificate is used for Plesk URL.
When opening Plesk using this URL (https://example.com), the page shows an SSL warning.
Plesk uses an SSL certificate configured at Tools & Settings > SSL/TLS Certificates.
Secure Plesk interface with a free Let's Encrypt certificate using the instructions from this KB article.