Jst bought 318is dont know radio anti-theft code

I just bought a 1991 318is and the antitheft is activated and i dont know the code. Does anyone know how to get it?? And how to enter it??

Pull the radio from the dash [leave it connected if you want] and write down the radio's serial number you'll find on the back of the unit [metal, plastic or sometimes paper tag]. The dealer can give the corresponding Radio Code in about a minute.