416.3. Activated abilities that Add resources and have the Reaction tag can be activated during the playing or resolution of other spells and abilities, any time that those spells or abilities require that 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.
416.5. This action is formatted as "Add [one or more resources]." e.g., "Add [2]." means "Add 2 Energy." e.g., "[E]: Add [Y]." means "Add 1 Power of the Order domain." e.g., "Add [1][G]." means "Add 1 Energy and 1 Power of the Calm domain."