Is it possible to install a certificate to secure FTP for a specific domain?
At the moment, it is not possibile to install a SSL certificate for a particular FTP domain, the certificate which is used for securing Plesk at Tools & Settings > SSL/TLS Certificates > Certificate for securing Plesk is also used for all FTPS connections to the server.
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To protect FTP connections follow the instructions from the article: How to secure Plesk login page with SSL certificate