There isn't a guide specifically for NodeBB, but you could follow this guide and use this page to find out how to get your certificate signed.
Let's Encrypt - Rulezz!
@sebastianneubert yeah one option is to do that. Another option is using a subdomain. Any reason why a subdomain wouldn't work for you?
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@julian any updates on this? This seems like an awesome feature to have.
Thanks, that did it!