So what I did was add the following code within my order confirmation page:
customerEmail = document.getElementById("Customer-Email").innerHTML;
document.getElementById('EMAIL-3').value = customerEmail;
This takes the customer email from their contact details (that’s the element with the ID “Customer-Email”, but check your own html) and then inserts it in to a newsletter subscription form (I used the Webflow component to do this, its ID was “EMAIL-3”.
Hope this helps!