Currently there is no way to tie JS to a keyframe in Animator.
One advanced way of accomplishing it, would be to add an hidden animation, with two keyframes, and listen for the 'animationEnd' event fired by the object when the animation is done and use that to fire your event.
A simpler approach would be to add a timeout to your Start Action of the scene. For example:
//get stage of first scene
var stageElement = document.querySelector('.AN-sObj-stage')
//create timeout at 1000ms (1 second)
//change the background color of the stage to red
stageElement.style.backgroundColor = "red";