Applied Costs are paid as the Game Action is performed. They do not use the chain and cannot be reacted to. 1. Abilities that Add resources with the Reaction keyword can be activated at any time when costs need to be paid. See rule 429. Add for more information.
§ 204.4.b Core Rules PDF (officieel)
§ 204 Costs paid in this way might be:
§ 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 b…
