How do I get my iPhone 8 to fast charge?
Yes, all of this year's newly released iPhones (8, 8 Plus and X) support fast charging. And no, there’s no setting you need to enable for it to work.

And, just to clarify, by fast charging you can recharge your iPhone up to 50% (not 80%) within the first 30 minutes.

However, for fast charging to work you need to use a charger that provides enough power for it to fast charge, and the chargers bundled with your iPhone only deliver 5 watts of power at up to 1 amp. By way of example, an iPad charger delivers 12 watts of power at up to 2.1 amps. Note: I am not saying that an iPad charger will fast charge your iPhone 8, although it would certainly charge it quicker than the standard iPhone charger.

Instead, I refer to what Apple says you need for fast charging your iPhone:


  Fast charging gives you a quick and convenient way to recharge your
  iPhone 8 or iPhone 8 Plus. Your iPhone fast charges up to 50 percent
  in 30 minutes when you use an Apple USB-C to Lightning cable and one
  of these adapters:
  
  
  Apple 29W, 61W, or 87W USB-C Power Adapter
  A comparable third-party USB-C power adapter that supports USB Power Delivery (USB-PD)
  


For more info refer to: Fast charge your iPhone 8 or iPhone 8 Plus.