There is a blue wifi (or network manager python program acçessory) icon in upper right corner, looks like an audio 776729 volume icon , in the MATE desktop, icon is different looking in XFCE desktop. Click on that icon to start/access networkmgr to enable wifi or wired network manager.
or open a terminal window, type 2 command killall networkmgr
networkmgr
"networkmgr" to start a second instance, since you killed the first instance. and can can work with the second instance, the same as the first, via clicking with the mouse pointer on the networkmgr icon.
what follows is the manual commands to achieve manual enable of wifi, from my memory. So I may have mistakes. network manager is easier to use.
ifconfig
ifconfig wlan0
ifconfig wlan0 scan #get list of available wifi network names
for networkname "abc123" and channel 6
ifconfig wlan0 ssid "abc123" channel 6
killall dhclient
text edit file /etc/wpa_supplicant.conf
wpa_supplicant -c /etc/wpa_supplicant.conf -i wlan0
dhclient wlan0
You asked about sign-in web page on a new wifi network at a public library. I did not answer your question specifically. Sorry, I gave a list of manual BSD commands to connect to a network. So open a web browser and see what happens, if you can login?
Read the fine manual commands:
man killall
man ifconfig
man wpa_supplicant
man dhclient