b - Why do I (as Dutch student) need a DigiD and how can I apply for my DigiD?

As Dutch student, make sure you apply for your DigiD in good time (at least one month before applying) because you will need this to be able to apply via Studielink. You can apply for your DigiD via https://www.digid.nl/en/.

If you do not have a Dutch home address and cannot apply for a DigiD, you will find more information and a step-by-step plan on this website.