Your username is your student ID or a variation of it. See the table below.

Username format
CampusNet    000123456Course content (assignments, quizzes, etc.)
Self-Service000123456Course registration, Transcripts, Final Grades
Microsoft 365 Online000123456@sebts.eduStudent Email, Office Online, OneDrive file storage, etc.

If you need to reset or change your password, please see this article