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Are you sure CasaOS doesn't have some built-in mechanism to get a cert? Their docs are nearly nonexistent, so you might want to check on their Discord.
If nginx is being used, one might think the --nginx plugin might work, but looking at the screenshot above and reading "Docker", this probably won't work.
Docker makes handling this kinds of things a little bit more difficult. Perhaps it's even wise to ask someone with more experience to do it for you.
If you want to make it properly you need certbot to issue commands against the docker socket, and that's a whole another can of worms because the certbot image uses busybox and there's no curl (but there's python, and requests); and having a container accessing the docker socket widens your attack surface enormously.
Quite fond of Caddy myself. But I still suspect CasaOS has some built-in cert management, even if their docs on the subject (as on almost everything else) are non-existent.
Edit: Yes, it does, in the form of Nginx Proxy Manager. Or, alternatively, using Cloudflare tunnels, but that would require you to be using Cloudflare. But the mechanics of installing and using that would be best addressed in the support channels for CasaOS, which AFAICT consists of a Discord server.
