A captive portal is a web page displayed to newly connected users before they are granted broader access to network resource. ESP8266, when configured as a WiFi access point, can serve a captive portal. On the other side of the spectrum, ESP8266 can be used as a WiFi client (aka STAtion), and it should be able to "click through" a captive portal as well.
Clearwave Solutions LLC provides WiFi service at my apartment. Their WiFi network shows a captive portal to each connected client once a week. The captive portal page has a giant Connect button, which instantly enables Internet access.
Having to press the Connect button every week is an annoyance on a computer or mobile phone. When it comes to little devices such as a temperature and humidity sensor, the existence of captive portal is a bigger problem because the ESP8266 does not have a web browser, so it is impossible for me press the Connect button. However, from network point of view, as long as the ESP8266 sends the correct packets, the WiFi gateway would think the button has been pressed.
To click through the captive portal without a web browser on the ESP8266, I just need to: