Implement GA cross domain tracking & e-commerce tracking

Hi,  

Understand that there are 3 ways to implement GA on Squarespace, 1) GTM, 2) External KPI Keys or the 3) Code Injection way. I need to satisfy 2 criteria, E-commerce tracking & Cross-domain tracking. 

The GTM way will not allow for e-commerce tracking so I can only consider 2 & 3.

Method 2 does not allow for me to edit the codes to include linker for cross-domain tracking, so I can only use method 3.

Now I am hitting a road block because I don't know how to use method 3, while having my e-commerce data tracked on GA (based on what I have tried, e-comms tracking only works with mtd 2)

This is the current gtag.js installed through code injection as shown below, does anyone know what I should do to integrate e-comms on GA through the gtag.js?

<!-- Global site tag (gtag.js) - Google Analytics -->

<script async src="https://www.googletagmanager.com/gtag/js?id=UA-XXXXXXXXX-X"></script>

<script>

  window.dataLayer = window.dataLayer || [];

  function gtag(){dataLayer.push(arguments);}

  gtag('js', new Date());

 

  gtag('config', 'UA-XXXXXXXXX-X', {

'linker': {

'accept_incoming': true

}

});

</script>