TaleWorlds Entertainment disabled the ability to paste code into the banner screen, so we need to install a mod to re-activate this feature.
(The instructions provided are pretty good.
It's easy to set and change your banner and colors in Mount & Blade II: Bannerlord, but did you know there's an unofficial, but super-easy way to create your very own banner? Compared to the options provided in-game, there's plenty of scope here to do pretty much whatever you want.