Hi,
is it possible to customize Square receipts to generate survey unique code, later print it on receipts with a promotional message?
Hi 
, unfortunately at this time customizing the receipt isn’t currently available. 
is there a plan to add that in the future? And, one more question- Is there a way to pass on restaurant customer information to a platform external to Square after payment is processed?
We’re constantly working to improve our features based on feedback like this, so I’ll be sure to share your request to the API product team.
If a customer is created and added to a Restaurants sale you can definitely pull customer data into an external database with our Customers API.
At what point the customer can be created? and is it possible to get customer information after post-purchase event?
A customer can be created prior to purchase and added to the sale so the customer information is available post-purchase.