You will first have to request the following information from your host:
1. Instructions on setting up a CRON job, provided that you know the command line to use. Normally you can find this information in Plesk's documentation. Unfortunately, we are neither the authors of Plesk nor your host so we can not provide you this information. If unsure, ask your host.
2. The full path to the PHP CLI (not CGI) binary. This is something which is dependent on your host. Since we are not your host, we don't know how your host's server is set up, therefore we can not provide you with this information.
3. The full filesystem path to your site's httpdocs directory, it's something like /home/myuser/httpdocs. Again, this is completely dependent on how your host has set up their server and, of course, we can't provide this information.
Then you can come back here and tell us what are the settings for #2 and #3 as communicated to you by your host. We will construct and give you the exact CRON command line you have to use. Then you will follow your host's instruction from item #1 to set up the CRON job on your site using the command-line we've given you.
If this all sounds like too much work, you can always use the third-party webcron.org service to set up scheduled backups. The downside is that it's not free. However, scheduling daily backups with webcron.org costs about $2 (two US dollars) per site per year. We have instructions in our documentation. If you want to follow this route and get stuck somewhere just tell us where you got stuck and we'll help you.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!