Below is the code you would enter in the additional content & scripts of then settings>checkout area of Shopify. This will allow you to create variables in google tag manager, that you can then use in the conversion tracking pixels....

Here is the best solution for Affiliate UTM Parameter for capturing CPS conversion without using any third-party tool. POSTBACK https://traqkar.com/p.ashx?o=548&e=290&f=pb&r=CAS...

Here are the details where you use some tricks to get a customer is new or old and pass it on instantly into the postback URL. Basic pixel looks like this  <!--Offer Conversion: _CPS --> ...