Exiling Flameskull as its triggered ability resolves isn't optional, but if Flameskull leaves the graveyard before the ability starts resolving, you won't exile it or the top card of your library.
You must still pay all costs and follow all timing rules for spells cast this way. Likewise, playing lands this way is subject to the normal rules for playing lands.
Adventures in the Forgotten Realms Promos
Adventures in the Forgotten Realms