Ah, I see; I tend to isolate all sites I trust in the same container and everything else goes in a temporary container so I mostly have one trusted container (along with one for each of Facebook, LinkedIn and Google).

To add a new site to a new container, I manually edit the file (accessed through the pencil button) and add the line with the proper syntax:

    !*.sketchy-website.tld , My Isolation Container

It allows me to define that I want that for all subdomains in the same workflow. But I have to admit it is far from easy for someone who is not already used to this.