I don't know for certain, but the top gear test track is won by golding the beginner special event at the top gear test track.
So logic dictates, that the nordschleife is unlocked on the Mercedes AMG beginner event by golding that, as the event takes place on the nordschleife. Take note that doing this might take a while because there are about 4 events within the beginner AMG event, an they aren't particularly easy to gold.