For each item in checkout basket - loop?

I’m using the Ez Marketplace - Shop v3.0 template. I can add multiple items to my basket, and when I click checkout, my current order is added to my placed orders, and I can see the items ordered on my order history. All of this is great.
However, at the point of checkout, before the current order is added to placed orders I want to send an external API call for each ordered item in the current order.
In pseudo-code it would be something like:
for each order item in current order
send external api request with order items’ fields
I just can’t work out how to create a for each loop.
Help is massively appreciated.

Could you schedule on the list?