You can only register online. At the bottom of the page that contains the course information, you will see a bar with the starting date of the course. Click to open this and you will see the link: “Register now”. You can also go to the Portal.
If you want to apply for a Dutch course preparing you for the NT2 exams with a DUO loan there is a different application procedure. You will first need an intake if it's your first course, you can schedule an intake interview here.