*** Update to the below solution, I thought it was working, it still may with some tweaking but I spent enough time with trial and error.
Best way really is to use the Cron Key generated from Blesta and Fetch Command within your plesk scheduled tasks
Works perfectly and allows you to use other services such as EasyCron
=== (As Nelsa mentioned within this thread)
You can set cron to https://****.com/clients/cron/?cron_key=your-cron-key
And check "fetch URL"
Change your-cron-key to key you can find in settings-->system-->automation-->update cron key
Albeit this question is a bit old, some may find this helpful.
Plesk Web Admin Edition running Blesta run every 5 min
Tools Settings / Scheduled Tasks (create new Cron Job)
/opt/psa/admin/bin/php -q/var/www/vhosts/YourDomain.xxx/httpdocs/index.php cron > /dev/null 2>&1
(Make sure there is "no" space between the php -q and /var as noted above)
(above assumes default location of public files httpdocs)
(Your location of php may vary depending on your hosting provider, or server install)
Use Cron Style for the "Run" : */5 * * * *
Will setup for every 5 min, every day, 365 days per year
Apply and Run (for test) should execute with green success pop up.
Dave / MailEdge