Siren of the Fanged Coast

For effects that check which player put counters on the entering creature, the player chosen to pay tribute puts those counters on it, not the creature’s controller.

The triggered ability will resolve even if the creature with tribute isn’t on the battlefield at that time.

The choice of whether to pay tribute is made as the creature with tribute is entering the battlefield. At that point, it’s too late to respond to the creature spell. For example, in a multiplayer game, opponents won’t know whether tribute will be paid or which opponent will be chosen to pay tribute or not when deciding whether to counter the creature spell.