Why am I being asked for my DNI while I'm not a Spanish tax resident?

Why do I have to enter my DNI?
Given the transaction reporting rules under the MiFID II Financial Markets Directive, all Spanish citizens, regardless of whether they are actually resident in Spain, must provide a DNI. Therefore, you must enter your DNI as a TIN (Tax Identification Number) together with your (tax residency) tax ID during your registration with DEGIRO.

How can I obtain my DNI?

In order to obtain your DNI, you can follow the steps on this page from Spanish Interior Minister Website.