Mageseeker Investigator

204.4. Applied Costs: These Costs are applied to one or more Game Actions, and typically take the form of a passive ability with a Cost within Instructions preceded by "must." Example: Mageseeker Investigator reads in part "Opponents must pay [A] for each unit beyond the first to move multiple units to my battlefield at the same time." This is an applied cost.

430.3. 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.