Press the button to the left of register / login.

Enter your RWTH Single Sign-On ID in the upper field and your PIN in the lower field.
Then press Senden.
If necessary, you can change your PIN at any time in the TK-Portal.

Note: The entry is made using the T9 principle as shown in the 2nd picture. In addition, the phone's navigation ring under the display is used to jump between the fields.