... as a doctoral candidate you have to be a member of the FU Berlin until you submit your thesis?
If you do not have an employment contract with the FU Berlin, you are required to enroll with the Student Administration Office. You can apply for de-registration from university in writing to the Student Administration Office at the end of the current semester at the earliest after submitting your thesis. You can find the application form for de-registration here.
In case you wish to de-register within the current semester, you must return the Campus Card to the Student Administration Office.