How to force a captive portal page to pop up (Like a Starbucks or McDonald's WiFi landing page)?

Normally, Safari provides a pop-up window for the public Wi-Fi's landing page: requesting the user to agree to TOS (Terms of Service).

Attempts to login to Wi-Fi were not successful because of the absence of the landing page:

• ifconfig revealed an IP address (172.31.99.185) was assigned to the laptop's Wi-Fi adapter
• attempts to ping 172.31.99.1 failed
• browsing to http://172.31.99.1/ failed
• browsing to http://www.google.com/ failed

Is there a Safari menu item or process that will call the TOS pop-up?

Go to http://captive.apple.com it’s the only way to ensure it pops up.

Apple now supports DHCP option 114 (as of iOS 14 iirc) to let network operators define a non-default captive URL trigger and suggests the full URL is http://captive.apple.com/hotspot-detect.html

The shorter URL works great for me as does this explainer of how to drop and signal auto log in on these captive networks:

• https://support.apple.com/en-us/HT204497

If the captive portal auto detection doesn't work, and hitting a common domain (like google.com), try a domain you don't normally visit (and hence won't be cached anywhere). Try borg.org, for example.

If that still doesn't get you the landing page, it's time for extreme measures. You can get more information about the DHCP config it got with ipconfig getpacket en0 (or en1, or whatever the Wi-Fi interface is); look especially at the "server_identifier" (the server that sent the info), "router", and "domain_name_server" lines. Try hitting those in your browser and see if any get you a landing page.

Still nothing? Next try host www.cnn.com x.x.x.x where x.x.x.x is the domain name server (if there are several, try with each). If you get back any IPs that weren't already on the list, try them in a browser.

BTW, the reason this sometimes doesn't work automatically is basically that it's all highly nonstandard. Captive portals are a hack that someone came up with to control access to a wireless network in a way that the standards didn't handle. Lots of other manufacturers copied the idea, but everyone did it a little differently, and it hijacked real web pages in the browser, and was generally a mess. So Apple built a system into macOS that'd try to detect captive portals and display the landing page automatically, in a way that didn't hijack the browser... but since there are so many variants on the idea, it's hard to build a system that handles them all. And then sometimes the local network admin sets things up weirdly, and... basically, it's a hack to get around another hack, and as a result there are a lot of ways for it to fail. Frankly, I'm kind of amazed it works as often as it does.