We can not advise on finding the Purchase event inside the pixel code as this is related to Facebook’s own tracking setup, and you may have set this up manually depending on your marketing stack.
Please review Facebook’s documentation HERE on tracking standard events such as Purchases. If you have access to the Facebook pixel code and are able to edit the Purchase event before placing it on your site, then you can move forward to the next steps.
In some cases, this may not be possible at all. For example, Shopify does not typically allow users to edit the Facebook pixel code installed on their site.