Whenever there are tools like this I usually just visit the site then unplug my ethernet/disconnect from WiFi. If it's actually client side JS it will work.
I'm pretty sure this is safe, but if there's a way to defer sending an HTTP request to after the page being closed...
If you do it from a private browsing window, maybe that would be fine. Maybe. (Otherwise the page could exfiltrate information via localstorage or something.)
At least with this one, you can audit the source code and build it for yourself[0]
0: https://github.com/OktaSecurityLabs/passprotect-chrome