§ 429.3 Core Rules PDF (officieel)

Activated abilities that Add resources and have the Reaction tag can be activated at any time that spells or abilities require resources be paid. Example: A player can add Energy and Power through any means before initiating the process of playing a spell. After initiating that process, in the Pay Costs step, they may activate Add Reactions to add Energy or Power to pay costs. Example: A player moves two units to a battlefield where their opponent controls a Mageseeker Investigator. Although they have no window of priority, they may activate Add Reactions to pay for the applied cost from Mageseeker Investigator, and those abilities finalize and resolve immediately.