Open the settings screen, paste your JavaScript, pick a page, save. The snippet is enqueued only on that page — not site-wide, not in the admin, not on the login screen. That’s the entire job.
Snippet managers exist and are great when you have a hundred snippets to track. If you have one — a chat widget on the contact page, a tracking pixel on a thank-you page, a custom analytics call on a single landing page — they’re more plumbing than the problem deserves.