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To do this, select the roster status that you wish to customise the notification for, and then enter the message that you want to be sent.
You can include template placeholder fields (e.g. '[First Name]', '[Event Name]', '[Event Location]'), which will contain information that changes will automatically change for each message - these can be added by either typing in the placeholder you want, or by selecting the placeholder from one of these dropdown menus: 
in the formatting menu.