You cannot set the drag to cast, you must set the cast control knob, or centrifugal brake.
With the lure hanging about 6 inches from the rod tip, press the cluth release button and allow the lure to drop to the ground.
Should a small overwind occur, tighten the cast control a bit. Should it drop very slowly, loosen a bit.
The ideal position to start casting is when the drop produce about one or two curls of line on the spool.
When you make the first cast, do not force it and control the spool with your thumb.
With more experience you can use less cast controll.
Remember to test it again with heavier lures and try not to cast very light lures in the beginning.
Enjoy and experiment with the best settings for yourself.
The manufacturers usually include a pamphlet explaining the settings of your reel.