First of all, welcome to the program!
Upon receiving your acceptance letter, look to see which university you have been assigned to, then follow the instructions listed in your letter on enrolling. Even though you applied to Freie Universität, your main enrollment can be with either Freie Universität or Humboldt Universität. The letter of acceptance is an official document that entitles you to enroll within a certain time frame. The enrollment has to be done directly with the admissions office before the deadline given in your letter. Please note that enrollment period at Freie Universität ends earlier than at Humboldt-Universität, but please check your letter for exact dates.
For information on enrollment you can contact firstname.lastname@example.org or email@example.com.
Once you have submitted all documents required for enrollment, you will receive your student ID from your main university. After receiving your student ID from your main university, it is important to enroll at your secondary university! After each enrollment, you should receive your account data for the online communication systems for the respective university.