This gives you the flexibility to ensure you actually end up studying where and what you want. (You can only apply to one medicine course.) However, you can change the courses you wish to apply for at any time before the enrolment deadline. You can apply for up to four courses at a time, of which only two can be subject to Numerus Fixus. Studielink is not the same as UCAS and it serves a slightly different purpose. Also, if you create an account and do not receive prompt acknowledgement of this, something will have gone wrong. Because of their approach to customer service, it is vitally important that you do not lose your login and password as it is extremely difficult to retrieve these. If you have a problem during the application process, your first point of contact will usually be the university to which you are applying.
However, their FAQs do not cover everything that could go wrong and there is no way to contact them directly except by post. You can find plenty of information about questions you may have on the Studielink website and the process usually works perfectly well. Every university has its own introduction page to Studielink and this will be where you first start to apply. Applications to Dutch universities should usually be made via Studielink. Studielink is the central application organisation for Dutch universities. Studielink - How to apply to a university in the Netherlands