Brainwash

• WWizards of the Coast2004-10-04
  Official Ruling
  Paying this cost is not an instant or any other kind of ability, it is an additional cost on the declaration of the attacker.
• WWizards of the Coast2004-10-04
  Official Ruling
  This ability is paid for during the declare attackers step of the combat phase.
• WWizards of the Coast2004-10-04
  Official Ruling
  If there are multiple combat phases during the turn, the attack cost must be paid each time if you want to attack with the creature.

Rulings via Scryfall