Where to place functions with custom build configuration? / Static Forms Plugin not working

Hey, I'm trying to use the static forms plugin (https://developers.cloudflare.com/pages/functions/plugins/static-forms/) and in theory I just need to add a /functions/_middleware.ts file, but I'm not sure where they should be. I have the attached build configuration, but should I create the functions directory in /, /frontend, or /frontend/build? In any case, trying it locally with npx wrangler pages dev build (running in /frontend and after npm run building manually) seems to find the _middleware.ts file in /frontend/functions because it fails to build if I don't have the plugin npm installed. However, even though it is supposedly building the function, it doesn't really do anything - when I visit the website, the form still has data-static-form-name and its method/action attributes has not been overwritten. Would appreciate a pointer on what I'm doing wrong. Thanks in advance!