Narset's second ability goes on the stack without a target. While that ability is resolving, you draw a card and may choose to discard a card. When you discard a nonland card this way, the reflexive triggered ability triggers and you pick a target to be dealt damage. This is different from effects that say "If you do . . ." in that you choose a target after looking at the card you draw and players can respond to the reflexive triggered ability knowing how much damage will be dealt.