If you want to limit who can install and use your published app, for example, your existing users or customers, you can do so by setting the 'From your landing page' option in your apps marketplace settings:
This will change the 'Add' button on your apps page in the marketplace to 'Visit site to add':
Then, on your website, you can put the Authorization URL behind your login so only users logged into your site, can authenticate / connect their Zoom accounts.
Once the user navigates to the Authorization URL, it will kickoff the OAuth flow, where they can authorize and install your app.
If you are looking to beta test your app with external Zoom users before publishing it to the Zoom App Marketplace, read here.