There are two types of codes you may use when registering for ThingLink: invitation codes and coupon codes.
These codes are 6 digits long and can be any combination of numbers and letters. They are used by students to join their teacher's groups, and by teachers that are joining a school or district account. If you are a teacher and you do not have a code, just leave it blank. If you do have an invitation code, use it on the registration page, not the payment page.
From time to time, ThingLink runs discount promotions. These codes can be any length and any combination of letters or numbers though they usually are not random and are instead specific words and numbers. If you have a coupon code, use this on the payment page, not on the registration page.