I hope you are doing good and welcome to the Shopify Community!
I am Mehak from MS Web Designer (Top Rated Shopify Certified Experts and eCommerce Consultant from Singapore).
If you cannot use the native Pinterest app for Shopify, you will need to install the code yourself, by following steps:
While steps 1 & 2 are generic (if you have any trouble, you should reach Pinterest support to help you add them), the step 3 (“Add to cart” code) is dependent on the theme.
1.Open the list of themes in your Shopify admin and, for the theme you want to add the code, click on “More actions” then “Edit code” (please note you need to have the theme purchased to do that):
2.In the “Assets” folder, click on “custom.js” file (note that depending on your theme and the custom code you have installed, you may have more files than what is shown in the screenshot below):