Inserting a GUID from a related entity into a custom field

I have struggled with this one to no avail, so far.  I have a workflow in CRM 2011 that sends an email to to members who have registered and paid for a class.  I want to include a hyperlink in the confirmation email that will take them to a public-facing folder where they can obtain their student workbook (50-60 page PDF).  I need to get the COURSE Guid from the Course table to concatenate that to the rest of the hyperlink.  Example:  www.association.com/studentworkbooks/{CustomCourseID(Class (Class))}.  

I added a custom field to the CLASS entity and am attempting to find a way to insert the COURSEID GUID into this field.  Can anyone give me a hand with this?

Thanks in advance for your help.

- Austin