How-to password protect sign-up on own Thinger.io web instance [CRITICAL]?

Cloud Integration
1

Hi, I successfully installed Thinger.io on a VPS.
Everything works, but anyone with the link could sign-up.
I found various ways how to protect the webpages or parts of it.

  1. Classic way with .htaccess
  2. More modern: Client side protection of a single HTTP page; in this case the /console/signup HTTP page; with https://www.maxlaumeister.com/clientside-html-password/

PROBLEM: THE SNAP FS is read-only so I cannot upload any (modified) file. Is there any other possibility?
Best, Robert