Peter here from Shopify Support!
I'm not too familiar with how this could be set up, however from what I can gather, entering your Google Analytics code into Online Store/Preferences should help with this. If you're unsure of how to get this added then feel free to check out this guide here that outlines the steps to add your GA account to your Shopify admin. From my additional searching, it appears that there may be an issue with the domain crossover from your third-party site and the Shopify checkout. For this, I'd recommend reading over this forum post here as it goes in depth on how you could solve this issue.
Hope that helps!
All the best,
Hi Shopify Staff,
This is just crazy to see that there is no proper answer provided by your staff to this burning issue. This has been an issue for more than 3 years. And all we see is reference to unrelated webpages or documents. How about you create a proper guide to do it rather than pushing into different rabbit holes.