I have created a form for my Shopify

Can you please help me make sure my form is loaded properly?  I copied the code from your Shopify to the Checkout code snippet block. (like where google analytics lives) 

I want the code to popup after payment processes.  Where to paste code?

I think you will have to integrate that into your Shopify template.

If you want the form to auto popup, you can use lightbox embed code. Guide: https://www.jotform.com/help/203-How-to-Auto-popup-a-Lightbox-Form

Anyway, here's a guide on how to embed the form into your Shopify store frontpage just in case this would help.

https://www.jotform.com/help/217-Adding-a-form-to-Shopify-Store-Frontpage