The extension already supports that, albeit in a semi-convoluted way: you have to visit the site to set a default container for it. If yours let's you edit it by hand, that is a pretty big quality of life improvement.
I think what you're referring to is the "Always Open This Site in .." option. That one assigns a website to automatically open in container. This on the other hand, assign a container to a default page. It's similar to how you have home page for your whole browser, but now for each container.