Unfortunately, there is not a 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 register.
If you have the Flipcause Webpack
You can create a password protected page through the Weebly website.
Once you create your member page using the instructions above, you can add the event campaign to the page.