I just deployed my side project and I'm adding the HTTPS certificates.
I was able to setup the certificate for the root domain but it looks like it's impossible to add it to my subdomain www - The UI's dashboard only let you setup one domain.
I ask that since I need two certificates - WWW and NON-WWW - to be able to redirect the WWW to NON-WWW.
You can create another certificate with a different domain, then create and activate a bundle for it as well. Nanobox doesn't limit the number of certificates you can add to any of your apps.
The process of adding a certificate is a bit involved, whether Nanobox is in the loop or not:
The first step is to create a certificate signing request (CSR), essentially a certificate with all the information about your domain (including the public encryption key), but no security. Nanobox refers to this as "creating a certificate", a process that starts every time at the "SSL/TLS Certificates" screen of your app's Admin tab.
The next step is to get that CSR signed, which is where all the security is layered in. Nanobox refers to this as "generating a bundle", since it pulls in the certificate used to sign yours, as well as any ancestor certs that may have.
The final step is to actually load that collection of certificates, also called a "chain of trust" or just "certificate chain", onto your server, where your site can use it to secure communications with itself. Nanobox calls this "activating the bundle", and it involves passing the certificate chain (the "bundle") to the app's load balancer, and telling it which domain to use that particular bundle for.
Here's where things get the most confusing the most often for most users with this question. The UI at this point mentions that "Only one bundle can be activated at a time" – it doesn't clarify that it means "one bundle at a time per certificate". Since you could, hypothetically, sign the same CSR multiple times with multiple Certification Authorities (CAs; LetsEncrypt is the most common CA for Nanobox apps), Nanobox supports loading multiple chains for a single CSR (or, to use the Nanobox naming, multiple bundles for a single certificate). But since SSL doesn't support multiple chains per request, you have to select a bundle to serve for the associated certificate, and that's all that bit means.
To add another certificate/bundle, go back to the "SSL/TLS Certificates" screen, by clicking that button on the left side of the page, then choose "New SSL/TLS Certificate" under any existing certificate(s) you've already added. The rest is the same process you already followed, above.
The workflow to add new certificates in Nanobox isn't the most friendly or obvious, though there are probably ways to improve on that, but hopefully that makes a bit more sense, now.