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From within your course click the Settings navigation tab, if you are within an activity you may need to expand the Course administration sub-menu. From within Course administration expand Users and then expand Enrolment methods. It's a bit hidden away but click on Click on the 'Edit' pencil alongside Self enrolment (Student). This will load a page of options, including where to set the Enrolment key. If no key is set, you may enter any key phrase or term into this area.
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