We don’t have an API option to start strapi in development mode with a server.js or app.js
strapi(/* {…} */).start(); ← This is basically the same as running npm run start or yarn start and what is required to use the development mode is basically npm run develop or yarn develop but there is no strapi(/* {…} */).develop(); option.
If Plesk allows you to run a custom command you could run it like this, but as mentioned earlier we don’t support nor recommend it.