Set a custom referer for an order in storefront
Use the ec.order.referer_id JS variable in your custom JS code on one of checkout pages (e.g. CART) to set a custom value. Set order referer to 'Amazon':
ec.order = ec.order || {};
ec.order.referer_id = 'Amazon'
Afterwards, this field will appear in order export via CSV, invoices and email notifications as a variable, and in the Ecwid REST API for orders as the refererId field.