App cannot be installed inside a custom site

I’m using v13 of frappe. Here, I’m creating a custom app and also a site. But while installing custom app into the site is not supported.
The command I’m using here is,

bench --site site_name install-app Custom_app

You cant create a site and install an app at the same time (To my knowledge). First use bench new-site SITENAME
After its completed and you have made sure the custom_app is in the frappe-bench/apps/… directory. You can now use bench --site SITENAME install-app custom_app.