Sorry, this form cannot be used to forward "www". You need to use the main forwarding page for "www" forwards.
However, looking at their main forwarding page, I don't see how to do that. It appears they allow forwarding https://johnfernow.com to https://www.johnfernow.com, but I don't see how to do the reverse — it appears to me that you can't do that, but maybe I should contact support or create a forum post asking for clarification.
I could set my custom domain in GitHub pages to www.johnfernow.com, but then http://johnfernow.com might fail (though perhaps I could forward it to the www.johnfernow.com page). However, it appears that I cannot generate an SSL certificate for both a www subdomain and an apex domain, according to this issue. It appears numerous decently-big sites either fail to load with a www in the URL or have a security warning because of this issue. Examples:
Currently, I don't know that I can fix this issue on my domain registrar, but perhaps if I switch to a different one (such as Cloudflare) I can see if they support forwarding www.johnfernow.com to https://johnfernow.com
https://johnfernow.com ✔ http://johnfernow.com -> https://johnfernow.com ✔ www.johnfernow.com ❌ (cannot find page, ISPs may automatically search for the URL, often trying (usually poorly) to auto-correct it).
I tried following NameSilo's sub-domain forwarding guide to have www.johnfernow.com automatically forward to https://johnfernow.com, but it displayed the message
However, looking at their main forwarding page, I don't see how to do that. It appears they allow forwarding https://johnfernow.com to https://www.johnfernow.com, but I don't see how to do the reverse — it appears to me that you can't do that, but maybe I should contact support or create a forum post asking for clarification.
I could set my custom domain in GitHub pages to www.johnfernow.com, but then http://johnfernow.com might fail (though perhaps I could forward it to the www.johnfernow.com page). However, it appears that I cannot generate an SSL certificate for both a www subdomain and an apex domain, according to this issue. It appears numerous decently-big sites either fail to load with a www in the URL or have a security warning because of this issue. Examples:
https://www.commonmark.org/
https://www.scikit-learn.org/
https://www.pytorch.org/
https://www.selenium.dev/
https://www.rpm.org/
https://www.webassembly.org/
https://www.wkhtmltopdf.org/
https://www.metaflow.org/
https://trailofbits.com/
https://www.microservices.io/
https://www.semver.org/
https://www.vue-chartjs.org/
https://www.flutterowl.com/
Currently, I don't know that I can fix this issue on my domain registrar, but perhaps if I switch to a different one (such as Cloudflare) I can see if they support forwarding www.johnfernow.com to https://johnfernow.com