How do I configure ProFTPd server to use TLS/SSL in plesk?

Plesk ProFTPd sever has compiled-in mod_tls.c module for SSL support ans SSL support can be configured in /etc/proftpd.conf for example in the following way:

<IfModule mod_tls.c>
TLSEngine on
TLSLog /var/log/tls.log
TLSProtocol SSLv23

# Are clients required to use FTP over TLS?
TLSRequired off

# Server’s certificate
TLSRSACertificateFile /usr/local/psa/admin/conf/httpsd.pem
TLSRSACertificateKeyFile /usr/local/psa/admin/conf/httpsd.pem

# Authenticate clients that want to use FTP over TLS?
TLSVerifyClient off

# Allow SSL/TLS renegotiations when the client requests them, but
# do not force the renegotations.  Some clients do not support
# SSL/TLS renegotiations; when mod_tls forces a renegotiation, these
# clients will close the data connection, or there will be a timeout
# on an idle data connection.
TLSRenegotiate required off
</IfModule>

As values in TLSRSACertificateFile and TLSRSACertificateKeyFile otion you can use as Plesk certificate, for example /usr/local/psa/admin/conf/httpsd.pem as your own one.

After that restart proftpd from xinetd : /etc/init.d/xinetd restart