Where to enable wireless charging on iPhone?

There's nothing to enable. You use a Qi compatible charger or a MagSafe charger from Apple (if you use MagSafe, you either charge your iPhone uncased or you must have a MagSafe compatible case on your iPhone). Simply place your iPhone on the wireless charger and it charges.

You do not need to "enable" wireless charging. It will happen automatically when your iPhone is properly placed on a Qi-certified charger.

The Apple Support article you arrived here from should have provided you with that information.

You're welcome.