If you are visiting the Netherlands from abroad to work or study and you will be staying less than 4 months, you are not required to register as a resident. However, in the case that you have to arrange any affairs with Dutch government agencies, you will still need a Citizen Service Number (Burgerservicenummer - BSN). This can be obtained by registering as a non-citizen (Register Niet- Ingezetenen - RNI) in the Personal Record Database (Basisregistratie Personen - BRP). You can contact the Municipality of Leiden, or one of 17 other selected municipalities to make an appointment to do this. These are: Alkmaar, Almelo, Amsterdam, Breda, Doetinchem, Eindhoven, Groningen, Goes, Heerlen, Leeuwarden, Nijmegen, Rotterdam, Terneuzen, The Hague, Utrecht, Venlo and Zwolle.
Registration as non-citizen is not offered at the ECL; you will need to call the Municipality of Leiden at +31 71 516 516 5, or one of the other municipalities mentioned above.
You will need to visit in person and present a valid proof of identity (a passport or identity card) and will be asked to provide your address outside of the Netherlands.