Simon 5 is coming soon!
Simon version 5 is currently in beta testing. It is a massive update, introducing a much-requested feature: syncing the Simon data between multiple Macs, plus a Simon Status service, a Link Checker filter, improved Find filter, iMessage and Speak Error notifiers, a new app icon, and much more.
Important: please note that Simon 5 requires a minimum of macOS 10.12 (Sierra), and will be a paid upgrade from Simon 4 after the beta. Purchasers since September 1, 2020 automatically get a version 5 license (that also works in previous versions).
Re: Testing a password-protected URL
So the web browser displays an authentication panel/sheet when you visit the site? That equates with the Username and Password fields in Simon.
When you say it doesn't work, are you checking the test after adding it, or doing a Preview while editing? The detailed entered in the Edit Test window aren't saved for use by the Preview until after you click OK, so checking from the Monitor window is the most reliable way to check pages with authentication.
I hope this helps; reply if you need more assistance.