Payment Confirmation link after order editing at Thank you page

[sravans]

Hi,

I’m trying to edit the order at the order status page to add a product with a discount (upsell). I see existing apps calling their server (for editing orders from their server to Shopify's) and then redirecting to the checkout confirmation page for customers to confirm the payment to finalize the edited order. 

I see below the URL format that the apps are redirecting to (which then directs to the actual order confirmation page)

https://shopdomain/{shop_Id}/order_payment/{order_id}?secret={idontknow}

Obviously, I have the shopId and the orderId from javascript. But how can we get the secret? I see this secret is changing per order so it must be something to query Shopify API from the server. I see no mention of this parameter in any of the docs. Any help or a clue is very much appreciated.

Thank you in advance!

Sravan