I will post steps to add cron job but keep in mind that most providers(webhostingbuzz and site5 have it in TOS if not changed)..don't alow CLI cron jobs under 30 minutes on shared hosting,you can see it and even set it but it won't execute....
Now log in to cpanel and click on cron jobs ,under common settings select "once per five minute" than under command paste your cron command,you can find cron command in blesta settings->system->automation
IF YOUR provider don't allow CLI cron jobs at 15 minutes than insted CLI set web cron and it will execute(confirmed for WHB),web cron is like this
Steps are same in cpanel just insted php command you can see in blesta settings->system->automation put this
GET https://yoururl.com/blesta_dir/cron/?cron_key=PUT_HERE_CRON_KEY
Just change url,blesta_dir and cron key Cron key can be found also at blesta settings->system->automation
Just to be clear unlike CLI cron, Web cron can be set on different server,hosting account you have with other provider,third party cron provider....etc and it will work same job.