The USB-C port is on by default. There are no settings for the USB-C, and you don't need to press any buttons to activate it.
USB-C port max output: 60W
Note: Make sure the Omni 20+ is powered on before plugging in your device and ensure you connect your USB-C cable to the Omnicharge before plugging it into your device. Otherwise, your device might recharge the Omnicharge.