Welcome to ESO addon development. You will be seeing a lot more limitations left and right from here on out.
I suggest you read the beginners guides on
the wiki. Your finding is one of the very first things mentioned in the
MyFirstAddon tutorial by Zenimax Online.
There are also a lot of great developer addons that will support you in your endeavors.
DevTools a collection of them, or simply check the
Developer Utilities section for some you like.
You can also check the UI source code
online or
download it. There is always something to learn from it.
If you have any questions, feel free to fire away. We have a really helpful community here.
Enjoy your stay.