You can install a Let's Encrypt certificate for free.
Login to Plesk as admin > Tools & Settings > SSL/TLS Certificate > click the Let's Encrypt button.
Fill the form. Make sure the domain name field matches the FQDN hostname (myhost.mydomain.com). The email address is required to receive expiration notices.
If you don't have a proper FQDN hostname, it may not work. In this case, make sure it's working before trying to set up a Let's Encrypt certificate. Otherwise, you may have to wait until the DNS propagates before trying again.
Once the Let's Encrypt certificate is set up, return to the previous screen (SSL/TLS Certificate) and click at the [change] button next to the Certificate for securing Plesk.
Select the Let's Encrypt certificate (server pool) and press OK.
The certificate will be installed and working, but your browser may still show Not Secure for some time. You can confirm if the certificate is installed correctly by opening the Plesk Admin Panel using another browser.