Place it at the reaction beat itself, after the reveal has landed on screen. For a bigger build, lead in with the anime wow sound effect for the stunned look, then follow this clip with the romance sound effect when the crush angle is the joke.
Über diesen Sound
This is a short cartoon ahooga horn blast, the old two-tone car horn that honks out in a goofy a-oo-ga rhythm in under two seconds. It feels silly and instantly over-the-top, turning a simple reaction into wide-eyed cartoon excitement, like a character has just spotted something and completely lost their cool.