I assume I'm supposed to enter my serial number, which according to https://support.trainzportal.com/index.php?/Knowledgebase/Article/View/121/5/registering-trainz I won't have, since I have TRS19.
Then you need to open a ticket with the helpdesk. Keep in mind that they're in a different time zone. As I type this, it's 11:56AM for me, which means it's 2:56AM tomorrow morning at N3V's headquarters, so if I open a ticket, they won't answer immediately. Link to Helpdesk: https://support.trainzportal.com/