How do I configure WiFi to log in to WPA at boot time, regardless of user being logged in?

When you are logged in and connected to the network, right-click the Network Manager icon. (It should be in the upper right of the screen.)

Click "Edit Connections..."

Find the connection you want to make available without login. Click it and click the "Edit" button.

Make sure the "Connect automatically" and "Available to all users" boxes are checked.

Now the connection will start up before anyone logs in and will be available to everyone on the system.

for "regardless of being logged in," you'll need to edit your /etc/network/interfaces file...

http://ubuntuforums.org/showthread.php?t=263136

That link describes the process pretty well...

iface wlan0 inet static
  address 192.168.1.15
  netmask 255.255.255.0
  wireless-essid my_essid
  gateway 192.168.1.1
  pre-up wpa_supplicant -Bw -Dwext -i$IFACE -c/etc/wpa_supplicant.conf
  post-down killall -q wpa_supplicant

An easier solution: add the following lines to /etc/network/interfaces

auto wlp1s0
iface wlp1s0 inet dhcp
  wpa-essid wifiName
  wpa-psk Password

I have tested it on 16.04 LTS. May work on other versions.

Source: https://ubuntuforums.org/showthread.php?t=1963404

For the sake of completeness, I'll also mention wicd, an alternative to Network Manager. I believe that if you configure wicd to connect automatically to a wireless network, it will happily do so at boot time.