Open a page on your live site that has a form and click into a field. Then open your website in Formleak: the setup check turns green as soon as data arrives.
Code Injection needs the Core, Plus or Advanced plan (or some older plans). The Basic and Personal plans don't have it.
Squarespace's checkout pages don't run custom code, so forms on checkout aren't tracked.
Based on Squarespace's own help pages: Squarespace: using code injection. Menus change from time to time; if something looks different, their help pages have the latest.