How to implement flashlight cursor into squarespace

Try adding this to Settings > Advanced > Code Injection > Footer

<style>
  /* Flashlight Overlay */
:root {
  cursor: none;
  --cursorX: 50vw;
  --cursorY: 50vh;
}
:root:before {
  content: '';
  display: block;
  width: 100%;
  height: 100%;
  position: fixed;
  pointer-events: none;
  background: radial-gradient(
    circle 10vmax at var(--cursorX) var(--cursorY),
    rgba(0,0,0,0) 0%,
    rgba(0,0,0,.5) 80%,
    rgba(0,0,0,.95) 100%
  )
}
</style>
<script>
  function update(e){
  var x = e.clientX || e.touches[0].clientX
  var y = e.clientY || e.touches[0].clientY

  document.documentElement.style.setProperty('--cursorX', x + 'px')
  document.documentElement.style.setProperty('--cursorY', y + 'px')
}

document.addEventListener('mousemove',update)
document.addEventListener('touchmove',update)
</script>