Hi there @AntonC!
Don here from Shopify.
As this sort of customisation would fall outside of our ability to provide theme support, you would just need to look at some of these options instead.
There are a number of page building apps in our app store that you can use to set up a landing page for your own store you can browse here.
If you have any questions about whether an individual app would meet your requirements (or any other support queries) you can contact the app developers using the contact details listed there on the app store page for that app.
As an alternative, you could look at having this custom feature added by a Shopify Expert.
You can browse the marketplace for custom features/code Experts here.
Finally, there are some paid themes that come with a landing page functionality of some sort or another, though as with the apps you would need to consult the developers of a given theme to be sure of what that theme can offer.
All the best!
Hi again there!
Don from Shopify once more.
As potential customisation like that would not be covered by our theme support, you would just need to check in with whichever Expert you intend to use to add this custom feature.
They will be able to tell you what is possible in terms of using the password page in this way, though it sounds more likely a standalone page would be used.
In any case, the developer you choose to work with will be able to field these questions and discuss the costs of such a project, so check out the marketplace linked above.