When do you tap Kaisa to add a Power when playing Hextech Ray in response to an opponent moving a unit to a battlefield?

Ruling: You tap Kaisa at the point when you play Hextech Ray, paying 1 energy and declaring the target. Kaisa's "add" ability can be activated during the process of paying costs for cards.

Sequence:

• Opponent moves into a battlefield and passes priority
• You gain priority and focus during the showdown
• You tap Kaisa while playing Hextech Ray at action speed, paying 1 energy and declaring the target
• Pass priority to opponent who can respond with a reaction or pass
• If opponent passes, Hextech Ray resolves

Nuances:

• "Add" abilities can be activated during paying costs for cards