Unfortunately, there is no way to create password-protected tickets in Flipcause. However, you can create a separate campaign that is not available to the public by creating a new campaign with just that one ticket type.
Remember, it is up to you where and how Flipcause campaigns are shared. This means that adding it to a private or hidden page on your website will keep it out of reach from other individuals you might not want to have registered.
If you have the Flipcause WebPack:
You can create a password-protected page through the Weebly website. Here's how - Weebly: Limiting Page Visibility to Members. Once you create your member page using the instructions above, you can add the event campaign to the page.