As I mentioned, it's for multiple different gmail accounts. Same url, different containers.

you might be able to trick around that if you can set up something that redirects. I have multiple bookmarks that go via the same URL via a redirect (it's an SSO login flow), but if I set the initial URL to be tied to a specific container, the entire flow remains in that container. So if you could set gmail.example.org to to redirect to gmail and gmail2.example.org as well, you could tie those urls to a container and use bookmarks. It's a bit roundabout, but it probably works.