Alright, this is a bit convoluted but bear with me.
I have GA integrated with my online store using the Shopify/GA integration. Great!
I'd like to create a GA event that fires whenever a user completes a checkout — my idea was to add this event to the confirmation page by placing it in Settings -> Checkout -> Order Processing -> Additional Scripts.
Furthermore, I'd like to send that event to a GA account that is different than the one I have integrated with my Shopify store. (Why? So I can run a Google Optimize test on my personal website, which is attached to GA account #2, and set a purchase on my Shopify store as the objective).
Here's what I have in that Settings -> Checkout -> Order Processing -> Additional Scripts field (where UA-XXXXX-Y is populated with the account ID for my GA account #2):