How to set up cronjob for Mailster plugin

Mailster plugin cron script uses JavaScript to process its tasks, while FastCron doesn't process Javascript in your script output. Fortunately, it supports WP Cron, so you should switch to using WP Cron first before setting up a cronjob.

Once you update the Cron Service to use wp_cron, you can set up a cronjob for WP Cron as instructed here.