Facebook pixels on Squarespace to track conversions for the shopify buy button

Hi - I followed Paul 2009 instructions to add a shopify buy button to my clients website as they wanted Klarna and finance options available.  They are now asking me to add their facebook pixel to the website to track conversions and have asked me to track add to cart.  

I assume this won't track as I have hidden the squarespace add to cart button.  can anyone recommend how we can track this please? thanks