No it is not the AFM. If you use an accountant to do your annual tax return your accountant should be able to provide them for you. You will need your TaxisNet ID and your TaxisNet password. As part of the process you will also need your AMKA number.
I'm afraid the whole process is in Greek and trying to use an English translation, e.g. with the Chrome browser, probably won't work. I described the full process in this thread
about 5 posts down the page. It looks complicated but it is fairly straightforward if you just follow the instructions step by step. You will need your mobile phone and email accessible as you go through the process since they will send you a verification email and a verification SMS, each with a one time use code, that will need to entered into a box on a pop-up form.
I got my first prescriptions through the service last week and it worked fine. Good luck.