@RicoDragneel: Go to soundtracks and press the "Add soundtrack" button. Then add a title, media and tags for the track. For media you need to select a service (e.g. Youtube) and enter the ID of the video (usually at the end of the link). You can link it with an anime by adding an "anime:ID" tag where you replace ID with the anime ID. Once you're happy with the result, press "Publish".