Put it at the start of the attack sequence, just before the beam hits, so the charge sets up the payoff. Follow it with Goku Drip for a bigger anime power-up moment, or follow a failed blast with Scream Meme when the attack backfires.
About this sound
A short, distorted shout stretches out the line 'imma firin mah lazer' in a slow, slurred, deep voice, as if a character is charging up for an enormous blast from an open mouth. It feels goofy and overblown, turning a serious attack pose into a joke. It comes from the Shoop Da Whoop meme that spread in the mid-2000s.