Place it just before the moment of transformation in your edit, then follow it with the Inception Horn as the power arrives to sell the scale of the change, and finish with Okay Let's Go when the powered-up character charges into action.
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This five-second voice line has Flowey, the golden flower from Toby Fox's Undertale and Deltarune, asking to borrow your power in a voice that starts sweet and turns urgently demanding. The false friendliness makes it funny and slightly unsettling at once, like a cheerful friend who clearly wants something from you. It comes from the games where Flowey first appears as the player's first meeting and a hidden antagonist.