Every Air Fryer manufacturer will have their own recommendatons for their particular air fryer, however the Philips Air Fryers recommend pre-heating the air fryer first.  I find that cooking times are more accurate if you preheat first.  The food also cooks faster because the temperature is already at the specified temperature.  If a cooking time specifies 10 minutes, and you don’t preheat the air fryer first, you may find the food is undercooked.  My recommendation would be to preheat the air fryer regardless of whether the manufacturer recommends it.  It certainly won’t do any harm.