Adding Custom Code to "Add To Cart" Button

Site URL: https://www.blairmade.com/

I am attempting to integrate Pinterest conversion tags with my Squarespace site. I've gotten two of them working, but still struggling with the AddToCart event. I tried adding code into the header to fire when the "Add to Cart" button was visible, but it isn't finding the button, even when it is there. Does anyone have any tips with how they've been able to integrate this pinterest tag? I am sure I am just missing something obvious, so any help is greatly appreciated. TIA!

window.onload = function() {
  test = document.getElementById('yui_3_17_2_1_1595252067143_172')
  console.log("Printing " + test);
  //alert(document.getElementById("yui_3_17_2_1_1595035499467_119"));
};