Block all websites except for 2

Use NX Enhanced to block all TLDs, then try to visit the website you want, check the logs, and whitelist the domains used. Depending on the websites, you might need to visit and check the logs multiple time because sometimes a script that was originally blocked tries to load yet another script. Alternatively, you can visit it on an unblocked profile with your browser's network inspector active and then sort the browser log by domain.

However, it's possible that the website you visit requires a vital resource hosted elsewhere, and if you whitelist that too, NextDNS can't block it when you're opening that other domain directly since it has no context, you'll want uBlock Origin for that.