Most recipes on this site indicate you should preheat the air fryer prior to starting to cook.  There are a couple of reasons for this.
Preheating times may vary between air fryer models. Most should take around 3 minutes to preheat, however, always consult your air fryer user manual or manufacturer to confirm.
Preheating prior to cooking allows consistency between recipes. If a recipe says cook for 5 minutes but the air fryer hypothetically takes 3 minutes to preheat with one model, and 4 minutes to preheat with another model, then preheating ensures you always start cooking at the specified temperature providing more accurate results.