Let´s Encrypt On Plesk 17.x with Ubuntu 16.4 (how to?)

That depends on the web server configuration. It's possible to configure web servers to do this, but it's not automatic as a result of having HTTPS or a certificate.

We develop https://certbot.eff.org/, which obtains certificates on the command line (and optionally also installs them in Apache or Nginx). You can also look at

for a list of available client software. Another popular and capable option there is acme.sh, which does not install the certificate for you.