How to secure a domain alias with an SSL certificate?
Is SSL possible on domain aliases?
To secure a domain alias with the Let's Encrypt certificate, follow the instructions provided in the article How to install a Let's Encrypt certificate for a domain alias.
To secure a domain alias with a certificate purchased from one of the certificate authorities, contact the certificate provider in order to add the alias name to the certificate as the subject alternative name (or SAN). Then, secure the main domain with this certificate as described in the article How to install SSL certificate for a domain in Plesk.