You don’t choose a card to cast as Whispersteel Dagger’s ability resolves. You simply get a permission to cast one later.

You’ll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.

In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any spells and/or permanents you control from Whispersteel Dagger’s ability are exiled.

You must follow the normal timing permissions and restrictions for the spell you wish to cast.