I don't have Forge specific advice, but to get a wildcard SSL cert you need:
- Either to buy a wildcard cert from a provider, and configure your server to use it;
- Or something like Caddy or Certbot that can automate renewing LetsEncrypt/other wildcard certs
- If you automate it, the software doing the automating needs to be able to integrate with your DNS provider, since wildcard certs now require DNS verification. This usually looks like configuring some API keys into the software that your DNS provider offers you.
Alternatively, there are services like https://approximated.app (I'm the founder) that can handle custom domain certs and routing for you if you'd prefer a paid solution.