429.2. Triggered and activated abilities that Add resources resolve as soon as they are finalized.

429.3. 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.

429.4. Adding is a Limited Action.