In order to connect to a WiFi client network, you need to connect to the AutoPi WiFi hotspot first. After that you can go to the local endpoint (local.autopi.io) where you will be able to see the device’s settings. Go to the WiFi tab. From there you are able to select a client network to connect to and add it to the networks list. After you’ve added it to the list, you’ll need to specify the password to connect to that network.
Make sure, if there is more than one network entry, to drag it to the top, as the topmost network has the highest priority and the device will try to connect to the first available one.