If you want to exercise a self-employed activity in Belgium as a non EU national, you must first obtain a professional card ("Carte professionnelle / Beroepskaart").
It is the place of business that determines the region responsible for submitting the application, not the self-employed person's place of residence. If you intend to work as a self-employed person in the Brussels-Capital Region, your application is examined by Brussels Economy and Employment (The Regional Ministry for Employment) and judged by a number of criteria, like the relevance of the project in terms of economic utility. The application must be in principle made from the Belgian embassy in your home country but can be lodged from Belgium if you hold a valid residence permit.
For the conditions and the procedure click here.
GOOD TO KNOW : hub.brussels is the Brussels agency providing a free universal public service for all businesses at every stage of their life cycle. Contact 1819 in order to get an appointment.