You can sign up using either your NFC ID or your Card Number. Both formats are accepted, with or without spaces and dashes.
Option 1: NFC ID
Your NFC ID is a 16-character hex string (e.g. E00485A4821B66D4). You can find it in the card0.txt file located in your spice installation folder. You have most likely generated it in spicecfg under the Cards section.
Option 2: Card Number
Your Card Number is the alphanumeric code printed on your virtual card (e.g. 3J6T 7PDG GUY7 981P). You can find it in-game after scanning your card, or on the Asphyxia profile page if your card has already been registered.