Dragonkin Berserker

• WOTC Official Reminder Textofficial rules
  Boast — Activate only if this creature attacked this turn and only once each turn.

• WWizards of the Coast2021-02-05
  Official Ruling
  Once you announce that you're activating a boast ability, no player may take actions until the ability has been paid for. Notably, opponents can't try to change the activation cost by removing your Dragons.
• WWizards of the Coast2021-02-05
  Official Ruling
  A boast ability can be activated at any point after the creature with that ability has been declared as an attacker. This can be before blockers are declared, after blockers are declared but before combat damage is dealt, during combat after combat damage is dealt, during the postcombat main phase, during the end step, or, in some unusual cases, during the cleanup step.
• WWizards of the Coast2021-02-05
  Official Ruling
  The cost reduction applies only to generic mana in the activation costs of boast abilities you activate. It can't reduce requirements of specific colors of mana.
• WWizards of the Coast2021-02-05
  Official Ruling
  If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.
• WWizards of the Coast2021-02-05
  Official Ruling
  If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.
• WWizards of the Coast2021-02-05
  Official Ruling
  If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.

Rulings via Scryfall