Create a file in the root of boot called: wpa_supplicant.conf (instructions below). Then paste the following into it (adjusting for your ISO 3166 alpha-2 country code, network name and network password):

country=US
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
    ssid="NETWORK-NAME"
    psk="NETWORK-PASSWORD"
}