The easiest way to roll with advantage and disadvantage is using the "a" and "d" flags.

Add 'a' after your dice roll to roll with advantage. This will discard all but the highest roll in the result. If you rolled one die the advantage option will automatically change it to two.

!r 2d20a

Disadvantage (d)

Same as the advantage option you can use disadvantage by adding a 'd' after the dice roll.

!r 1d20d

See Dice Rolls and Messages for more info on advantage, disadvantage and other functions.