If you do not have https configured already (you do if you have done a standard install recently) see Setting up Let’s Encrypt as your first step,
Step 1 - letsencrypt
It’s pretty simple really, add something like the following to your app.yml
hooks section:
after_ssl:
- replace:
filename: "/etc/runit/1.d/letsencrypt"
from: /--keylength/
to: "-d second-domain.com -d www.second-domain.com -d other-domain.com -d www.other-domain.com --keylength"
If you just want to have https configured for a second domain that will redirect to the correct one (e.g., your forum is community.example.com
and you want people who visit https://forum.example.com
be redirected without a certificate error), that’s all you need.