wpa_setup.sh

sudo -E bash -c 'wpa_passphrase $YOUR_WIFI_NAME >> /etc/wpa_supplicant/wpa_supplicant.conf'
# enter your password into the prompt

/sbin/wpa_supplicant -B -c /etc/wpa_supplicant/wpa_supplicant.conf -i $YOUR_WIFI_DEVICE

/sbin/dhclient $YOUR_WIFI_DEVICE

Using either DHCP or a static config (doesn't matter which)--AND assuming your wifi worked during install--make your /etc/network/interfaces look something like below (for wlan0 should match the name of your wifi card listed under ifconfig -a e.g. your detected wifi card could be nicknamed eth1 by the OS for all I know.):

 auto lo iface lo inet loopback     
 auto wlan0 iface wlan0 inet dhcp    
 wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

To configure wpa_supplicant use the command (Referenced in the config above)

wpa_passphrase "YOUR_SSID" SSID_PASSWORD | sudo tee /etc/wpa_supplicant/wpa_supplicant.conf

Next, create a new executable script named iwconfig (you can name this script anything really, "iwconfig-default-ssid", perhaps?--I just made it short for the example):

sudo touch /etc/network/if-up.d/iwconfig && sudo chmod 700 /etc/network/if-up.d/iwconfig && sudo ln -s /etc/network/if-up.d/iwconfig /etc/network/if-pre-up.d/iwconfig

Now edit /etc/network/if-up.d/iwconfig and add the SSID you want Ubuntu Server to connect to on startup:

#!/bin/sh
iwconfig wlan0 essid "YOUR_DEFAULT_SSID" mode managed

Now bring ifdown (if you haven't already), then ifup, and you should be golden now and when you reboot (as long as you're near your SSID.)