The one with www.forum. You want to replace A to CNAME record.
www.forum → forum.alltiago.com
Remember, no proxy!
The guide is for additional domains!
So, you want to add www.forum.alltiago.com instead.
forum.alltiago.com is already being handled by default.
Otherwise, the location and indentation seems correct.
You have nothing to do! Just add the additional domain, rebuild.
Let’s encrypt will issue a certificate automatically.