Welcome to PhotoStructure, @Thunderclap !
PhotoStructure doesn’t have any authorization or authentication yet: that will be built out for sharing (be sure to vote for that feature!)
Most reverse proxies support HTTP authentication, though, so for now, use that. I wrote instructions on how to do this with Caddy here: https://photostructure.com/faq/remote-access/#example-caddy-configuration.
Cloudflared is also an option: https://forum.photostructure.com/t/using-cloudflare-tunnel-with-photostructure/776