How would I give an Arduino-based product WiFi credentials?
Most WiFi chips/modules these days worth paying for can function both as a WiFi client or as an AP.

So, what you would do is when the device is unconfigured, put your WiFi module into AP mode, and serve up a lightweight webpage hosted from your device. The owner connects to the devices WiFi network, and accessing the device through that webpage, they can setup their wifi information. Once configured, you switch the WiFi back to client mode, and connect to the provided Wi-Fi network.