How to grant permissions to a specific website only for a Safari extension?

I was quite elated to hear that Apple introduced a feature to give access only to a limited set of websites for installed Safari extensions. This is also denoted in the macOS Big Sur changelog:

Grant extensions website access

Keep the websites you visit and tabs you use even more private. Safari asks you which websites a Safari extension can work with — and you can grant access for one day or all the time.

However I can't find how to do it in the new version of Safari and macOS Big Sur. In the Extensions preferences it's all the same - only the list of installed extensions and permissions they require (without any way to configure them). Where can I find this precious hidden feature in Safari?