It is something that many users are asking for, but at the moment, there is not official, easy-to-use solution to secure the Plesk host by a Let's Encrypt certificate. You can use the Let's Encrypt client of the console to generate a certificate and then install that cert manually into Plesk, but it requires some in-depth knowledge. Maybe waiting on the official solution is easier? For Q2 2017 Plesk had announced that they are planning a big update for Let's Encrypt interface, so we can hope that this will include a solution to protect the panel by a cert, too.