Survey needed : just spoted this quikly but seam's here there is a building passage from Rue de Beughem  going to Justice de paix de Schaerbeek 1e canton. Signs indicate an entrance but don't know if it's the public entrance or just a private service drive.
Official address seams to be on Rue Royale : 
https://www.tribunaux-rechtbanken.be/fr/justice-de-paix-de-schaerbeek-1e-canton