To log in to Moodle as an external student, you need a user id on the central UHH-Login service provided by the University of Hamburg.
In order to participate in a course in Moodle, teachers could order short-term IDs and provide the login information to the external students. Visit the website of the Regional Computer Center (RRZ) and follow their instruction: https://www.rrz.uni-hamburg.de/en/kennung-authentifizierung/kennung-beantragen/kennungen.html