The Beleaguered Boxer can't be your Commander unless your deck is 100 cards and originally built out of no more than 36 booster packs, plus basic lands.
When The Beleaguered Boxer enters the battlefield, open a Magic booster pack. Remove up to three cards in your hand or graveyard from your deck permanently. For each card removed this way, add a card from that pack to your hand. Those cards are now permanently part of your deck.