Every student, employee or guest of TUM gets a so called TUM ID ("TUM-Kennung") from TUMonline. It is part of the TUM account. Its format is "no34yif" (consonant, vocal, digit, digit, consonant, vocal, consonant). This loginname is - other than your e-mail-address email@example.com or firstname.lastname@example.org - unchangeable.
You can find your own TUM ID in TUMonline: log into TUMonline and click on "TUM ID" in your business card.