Custom Resource Alert - 2 week Reminder for Voluntary Assignments

Summary:

Is it possible to create a custom resource alert for voluntary assignments?

Content (required): I have created an alert to remind learners 2 weeks before required assignments are due based on the due date. However, my customer would like a reminder sent to remind learners of a course starting which they will likely have booked the offering themselves and it is therefore Voluntary. What attribute could I use to do this? I was hoping I would see offeringStartDate but I cant see it. Any ideas much appreciated.

Version (include the version you are using, if applicable):23A

Code Snippet (add any code snippets that support your topic, if applicable):