Recently I was looking into how to make subdomains on Neocities. I could not find any resources, information or even a concrete answer on if it was possible or not online. So I ended up taking the plunge on a supporter membership, hoping it was doable. After a little bit of fiddling, I was indeed able to get subdomains working. It's not a difficult process, though it is a bit of a hacky workaround. In order to do this, you need a custom domain name as well as a Neocities supporter membership.

Steps

First, you'll want to go to your domain registrar's website and enter the settings for your domain. On Namecheap, this can be accessed by going to your Domain List -> Clicking on "Manage" by your domain's name -> Advanced DNS. Once there, create a new host record. It should be an A record, the name for your host should be the desired title of your subdomain, and the IP should be set to the Neocities host IP, which is "198.51.233.1" per the Neocities custom domain page.

Once you've saved your host record, go the Neocities website. Enter your account settings and create a new website. Your new site's name doesn't have to match up with the subdomain's name. Once you've done that, click on "Manage Site Settings" for your newly created site. Click on the custom domain tab. Once there, scroll to the bottom of the page and insert the name of the subdomain into the text box. The format will be the name you entered into the "host" box on your A record followed by the name of your existing domain. In my case, this comes out to testdomain.maydb.net

Concluding Notes

After that, well, you're done! One more thing to note about this process; your subdomain will exist as a separate website on Neocities, meaning that they have to be edited separately as well. On the website, all you have to do is switch between sites but this also means that you have to use the Neocities code editor. If you push updates through the CLI, you'll have to logout, run a command to get the login screen to appear and then login to your desired site every time which is a bit of an annoyance. Your password will be the same as your main site's, FWIW. Threw me for a loop at first.

Hopefully this is helpful to someone out there!