How to get the captive portal's URL which I am currently connected to? I just connected to a WiFi network which uses captive portal login page. I would like to view the captive portal's URL or IP address.

How do I do that?

I have tried clicking forget network on the WiFi in order to get the login page again, but it just connects without triggering the login screen, I think it saves the login info upon correct login.

The second thing I have tried is to wipe the cache and data of the CaptivePortal app from system settings, with no luck.
From this answer on Stack Overflow, Captive Portal is triggered only when you're trying to make a request.

So the answer is: Open an arbitrary web page in your favorite browser, and you'll get redirected to the Captive Portal login page.

From my own experiences, I recommend that you visit a website that does NOT use HTTPS, as encrypted connection may interfere with the redirection (Captive Portal redirection is in fact a man-in-the-middle).