The following is a step by step guide to retrieve Moodle's Self Enrollment Key of any moodle course:
- click the course name
- click turn editing on button
- click Course menu under Course Administration menu
- click Enrolment methods menu
- click Self enrolment (Student) edit icon
- Enrolment key is shown under Self Enrolment
- click Unmask check box to show the Enrollment key
Download the PDF file as reference. Note that the above applies to course creator/faculty/teacher role.