This is not possible via the online portal. You must fill out an examination registration form [1] and bring it to the student service ("Studierendenservice", mailbox). A few days later, you can pick it up there signed and bring it to the administration office of the institute conducting the examination. 
After the examination, the institute will send this form back to the study office (Studienbüro).

[1] https://www.sle.kit.edu/downloads/Sonstige/Pruefungszulassung-Erstversuc...

Kategorie: Studium - Alle Studiengänge