Consider the following game state.
- I have a Sky Skiff in play. It is not currently a creature.
- I cast Herald of the Fair
- It has an ETB trigger that targets a creature.
Can I tap the Herald of the Fair to Crew the Sky Skiff to make it a creature so that I can target it with the ETB trigger? Or do I have to choose the target as soon as the trigger goes on the stack and have no time to respond to the trigger to make the Sky Skiff a valid target?