How to add Advertising Pixels to Checkout Page to measure purchases better rather than product page

Hello, 
We are running advertising campaigns to our specific products for sale.....we want to measure the success of our advertising through having the pixels firing from FB / Reddit. However the results aren't clear due to the pixel not being present on the check out page or thank you for purchase page.....If the pixels were active on these pages rather than the shop page I think we would get better data? Does anyone know how you can add pixels to the check out page or thank you for purchasing page after order? is this possible?