There are three possibilities:

1. If it is a course at the FU, you can contact Ms. Zech / Ms. Nardozza (geopruef@zedat.fu-berlin.de) from the Examination Office (German: Studien und Prüfungsbüro) so that they can manually enter the course you have taken into Campus Management. The lecturer of your elective module can then enter your grade or a "pass/fail" as per usual, after the course has been completed.
2. Alternatively, you can also have a written certificate of successful course completion, the so-called "Leistungsnachweis" or “Teilnahmebestätigung”, handed out on paper. Especially if you have taken a course outside the FU, this is the usual procedure. The certificate is usually issued by the faculty where you are taking your course.
3. If you have completed your interdisciplinary elective modules during a semester abroad (i.e., Erasmus, Direct Exchange), your final transcript of records, which shows the number of credits achieved, the names of the modules completed as well as your final grades will serve as your proof of successful course completion. In the case that your transcript does not show a) credit points in terms of ECTS or b) grades that correspond to the German grading system used at the FU, you will have to additionally submit proof of translation of your credits and grades achieved into the German FU system. We recommend that you use the FU-internal grade translation guidelines; these can be found on the FU website here, available unfortunately only in German language, as a link embedded under “hier” in the following sentence on the website (please note that this link can only be opened when logged into the FU wifi or when using a VPN): “Eine Empfehlung zur Umrechnung im Ausland erbrachter Noten finden Sie hier. Bitte beachten Sie, dass diese Tabelle nur zur FU-internen Nutzung vorgesehen ist und daher nur aus dem FU Netz abrufbar ist.”