It’s free, just enter the code at the link and you can find it in the “collections” menu on your character (F1 menu) under Flair → Emblems → General. It’s called “Be True.”
Unlock it using code ML3-FD4-ND9 on our code redemption site.
The code is unlimited use and unlimited time as far as I know. There are codes from 2017 that are still valid.