Wait, do you mean the screenly screen that shows up when your device is not configured for wifi? this doesnt work for you? how so?
Unless you connect to that wifi, and then type in your URL browser http://screenly.io/wifi , and configure the SSID of your wifi there, this will always come up as the AP mode, unless you connect an ethernet cable which will then prevent this from coming up since the ethernet connection (if properly working) will establish the connection thus the RPi no longer needs you to give it wifi.
if you want to manually configure the wpa_supplicant.conf then you need to edit this file:
/etc/wpa_supplicant/wpa_supplicant.conf
https://wiki.archlinux.org/index.php/wpa_supplicant
example below is what you keep seeing?
