I recently opened a store, installed Facebook Pixel on the website and added events using Events Setup Tool.
However, I just noticed that for Purchases event, I've added Complete Purchase button on the checkout page and it redirects to PayPal. I don't have Shopify Payment Gateway as I'm from a country which doesn't support here. PayPal is the only solution.
Firing a Purchase event for Complete Purchase Button is not the best indication for a Purchase as a customer may or may not complete the purchase (can drop off).
Now I want to add FB Pixel to Shopify Thank You page so that I can track actual purchases. How can I do this?
You can either setup your whole Facebook tracking through the Facebook channel (see here), or add a Purchase event by going to Settings > Checkout, and placing your code inside the Additional Scripts section.
So without having to install the code separately, once I install Facebook app on Shopify, it automatically creates a Pixel and gets it installed on the Thank You page as well so that it tracks all conversions correctly?
Indeed it does, once you connect your Page/Ads Manager to the Facebook channel. It was launched recently, so the process might be a bit buggy, but if you set it up you don't need any other pixel code on your store, all of the standard commerce events will be sent automatically to your pixel tracking.