Does Ezreal Prodigy's optional additional cost discount apply to Draven Vanquisher's activated ability that allows paying Fury for +2 power?

Ruling: No, Ezreal Prodigy's discount does not apply to Draven Vanquisher's activated ability. Optional additional costs only apply to costs paid when playing a card, such as Accelerate, Repeat, or abilities that specify "when you play me, you may pay an additional cost."

Nuances:

• The ability must explicitly state "as an additional cost" or be inherent in rules text like Accelerate and Repeat
• Activated abilities that trigger after a card is already in play are not optional additional costs