Unfortunately, this is the problem with website builders.
Most website builders offer small areas where you can insert some limited amount of code, but that's about it. You don't really have full ownership of your website or your code. And everything is managed, updated and filtered through a 3rd party. Website builders are great for hobby websites or people who are not very serious about marketing online.
If you're a business, I would recommend Wordpress. If you're just dabbling in web design for fun, you will have to stick within the parameters of a rigid framework.