The are only straights in drag events,so you do not need to think about turns.
Make sure you use full gears in the drag.
Choose soft tyre (if possible) to increase traction.
To increase the acceleration,turn front splitter to 0.
Reducing rear downforce will bring higher top speed,but will cause more slipping at start.
The camber(negative or positive)will decrease longitudinal grip,so make camber as close to 0 as possible.
But you should watch the actual camber in car status,not setup panel.
In this image,the LF/LR camber in setup panel is 0 but the actual camber is xxx,that's because of the weight of the car changes camber.
Set to 1 liter to reduce weight.
Make your car as close to the ground as possible.
Traction control(TC)should be turned off.It'll lag much.

I'd have to say drag is quite difficult for people with slow response like me even with perfect setup,so there is a "dirty" way to beat AI in drag mode:

• Choose 1st gear if manual or N gear if semiautomatic/sequential.
  
• Rev your engine
  
• Wait your opponent to rev engine.
  
• When you hear opponent's engine roars,say"21",then launch your car.

But I think it's not a "dirty" way,AI's response is much faster than us after all.
Original video:
https://www.youtube.com/watch?v=PxcTTC-pMdA