How to get the captive portal's URL which I am currently connected to?
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).